build: Add $(top_srcdir)/src/compiler/spirv to AM_CPPFLAGS

build: Add $(top_srcdir)/src/compiler/spirv to AM_CPPFLAGS

Generated C files try to include spirv_info.h.  For in-tree builds,
the header is in the same directory, so it just works.  For out-of-tree
builds, we need to look for it in srcdir rather than builddir.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101831
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>