Previous iOS/Xcode fixes for the ARM NEON optimizations moved the test
on __ARM_NEON__ from configure time to compile time.  This breaks symbol
prefixing because the definition of the special png_init_filter_functions
call was hidden at configure time if the relevant compiler arguments are
passed in CFLAGS as opposed to CC.  This change attempts to avoid all
the confusion that would result by declaring the init function even when
it is not used, so that it will always get prefixed.