[Xperia Z3 Compact Original Android Development] .:B:.:
OK I found it. Turns out it was duplicate code in fs/fs_struct.h. Now, I had to mess with INSTALLED_KERNEL_HEADERS again (holy moly) and I keep bumping into this:

FAILED: /var/data/code/android/lineage-15.1/out/target/product/z3c/obj/EXECUTABLES/tfa9890_amp_intermediates/tfa9890.o
/bin/bash -c "PWD=/proc/self/cwd /usr/bin/ccache prebuilts/clang/host/linux-x86/clang-4053586/bin/clang -I device/sony/msm8974-common/include -I device/sony/shinano-common/include -I device/sony/z3c/include -I INSTALLED_KERNEL_HEADERS/include -I device/sony/shinano-common/tfa9890 -I /var/data/code/android/lineage-15.1/out/target/product/z3c/obj/EXECUTABLES/tfa9890_amp_intermediates -I /var/data/code/android/lineage-15.1/out/target/product/z3c/gen/EXECUTABLES/tfa9890_amp_intermediates -I libnativehelper/include_deprecated \$(cat /var/data/code/android/lineage-15.1/out/target/product/z3c/obj/EXECUTABLES/tfa9890_amp_intermediates/import_includes) -I system/core/include -I system/media/audio/include -I hardware/libhardware/include -I hardware/libhardware_legacy/include -I hardware/ril/include -I libnativehelper/include -I frameworks/native/include -I frameworks/native/opengl/include -I frameworks/av/include -isystem /var/data/code/android/lineage-15.1/out/target/product/z3c/obj/include -isystem bionic/libc/arch-arm/include -isystem bionic/libc/include -isystem bionic/libc/kernel/uapi -isystem bionic/libc/kernel/uapi/asm-arm -isystem bionic/libc/kernel/android/scsi -isystem bionic/libc/kernel/android/uapi -c -fno-exceptions -Wno-multichar -ffunction-sections -fdata-sections -funwind-tables -fstack-protector-strong -Wa,--noexecstack -Werror=format-security -D_FORTIFY_SOURCE=2 -fno-short-enums -no-canonical-prefixes -DNDEBUG -g -Wstrict-aliasing=2 -DANDROID -fmessage-length=0 -W -Wall -Wno-unused -Winit-self -Wpointer-arith -DNDEBUG -UDEBUG -fdebug-prefix-map=/proc/self/cwd= -D__compiler_offsetof=__builtin_offsetof -Werror=int-conversion -Wno-reserved-id-macro -Wno-format-pedantic -Wno-unused-command-line-argument -fcolor-diagnostics -Wno-expansion-to-defined -fdebug-prefix-map=\$PWD/= -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Werror=date-time -nostdlibinc -msoft-float -mfloat-abi=softfp -mfpu=neon -mcpu=krait -mfpu=neon-vfpv4 -D__ARM_FEATURE_LPAE=1 -target arm-linux-androideabi -Bprebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9/arm-linux-androideabi/bin -std=gnu99 -mthumb -Os -fomit-frame-pointer -fno-strict-aliasing -fPIE -D_USING_LIBCXX -DANDROID_STRICT -Werror=int-to-pointer-cast -Werror=pointer-to-int-cast -Werror=address-of-temporary -Werror=return-type -MD -MF /var/data/code/android/lineage-15.1/out/target/product/z3c/obj/EXECUTABLES/tfa9890_amp_intermediates/tfa9890.d -o /var/data/code/android/lineage-15.1/out/target/product/z3c/obj/EXECUTABLES/tfa9890_amp_intermediates/tfa9890.o device/sony/shinano-common/tfa9890/tfa9890.c"
In file included from device/sony/shinano-common/tfa9890/tfa9890.c:33:
device/sony/shinano-common/tfa9890/tfa9890.h:18:10: fatal error: 'sound/tfa98xx.h' file not found

The Makefile looks like this:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_SHARED_LIBRARIES := \
liblog libutils libcutils

# Was the following
#LOCAL_C_INCLUDES += $(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ/usr/include
#LOCAL_ADDITIONAL_DEPENDENCIES += $(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ/usr

LOCAL_C_INCLUDES += $(INSTALLED_KERNEL_HEADERS)/include
LOCAL_ADDITIONAL_DEPENDENCIES += $(INSTALLED_KERNEL_HEADERS)

LOCAL_SRC_FILES := \
tfa9890.c \
tfa9890_amp.c

LOCAL_MODULE := tfa9890_amp

LOCAL_MODULE_TAGS := optional

include $(BUILD_EXECUTABLE)

Thanks.