Move llvm_init_inner inside #ifdef ENABLE_LLVM

This should fix c30a3e46d2f94682b4ec1cac42b7c7ddb0a39059 failing to link properly with ld 2.22 and GCC 4.9.1, on Linux, with non-LLVM builds.

./.libs/libmini.a(libmini_la-mini.o): In function `llvm_init_inner':
mini.c:(.text+0x1182b): undefined reference to `mono_llvm_init'
/usr/bin/ld: .libs/libmonoboehm-2.0.so.1.0.0: hidden symbol `mono_llvm_init' isn't defined
/usr/bin/ld: final link failed: Bad value