Makefile.inc: Use no-packed-not-aligned only with GCC

Clang doesn't know the warning `packed-not-aligned`, so only add it
when GCC is used.

    error: unknown warning option '-Wno-packed-not-aligned'; did you \
    mean '-Wno-over-aligned'? [-Werror,-Wunknown-warning-option]

Change-Id: I86ee12a12fc24a0b8b92c4a0e665103ee4c4003d
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: https://review.coreboot.org/26879
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed