Mk/Uses/vala.mk: fix when COMPILER_TYPE=gcc

gcc does not support -Wno-error=incompatible-function-pointer-types.
Only enable the warning for clang to fix various ports that fail to
build with error messages like

cc1: error: '-Wno-error=incompatible-function-pointer-types': no
option '-Wincompatible-function-pointer-types'; did you mean
'-Wincompatible-pointer-types'?

MFH: 2023Q4