lib/zlib_inflate/inffast: check config in C to avoid unused function warning

lib/zlib_inflate/inffast: check config in C to avoid unused function warning

Building Linux for ppc64le with Ubuntu clang version
12.0.0-3ubuntu1~21.04.1 shows the warning below.

    arch/powerpc/boot/inffast.c:20:1: warning: unused function 'get_unaligned16' [-Wunused-function]
    get_unaligned16(const unsigned short *p)
    ^
    1 warning generated.

Fix it by moving the check from the preprocessor to C, so the compiler
sees the use.

Link: https://lkml.kernel.org/r/20210920084332.5752-1-pmenzel@molgen.mpg.de
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>