enc_mips32: workaround gcc-4.9 bug

avoids an ICE with NDK r10b + NDK_TOOLCHAIN_VERSION=4.9

In function 'SSE16x16':
enc_mips32.c (684) internal compiler error: Segmentation fault

Change-Id: I1a3d33c0a9534c97633ab93bcdf9bf59d3a7e473