android: gallium/auxiliary: fix building error in Android 7.0

android: gallium/auxiliary: fix building error in Android 7.0

Conditional libLLVMCore static library dependency is added,
for the case when MESA_ENABLE_LLVM is true

Fixes the following building error with Android 7.0:

In file included from
external/mesa/src/gallium/auxiliary/gallivm/lp_bld_misc.cpp:62:
...
external/llvm/include/llvm/IR/Attributes.h:68:14: fatal error:
'llvm/IR/Attributes.inc' file not found
    #include "llvm/IR/Attributes.inc"
             ^
1 error generated.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>