configure.ac fix build error for config.h in autotools

After the commit 0926f0af ("meson,configure: include config.h
automatically"), there is build error for autotools because
config.h is not included. Fix the error by adding "-include
config.h" to CPPFLAGS instead of CFLAGS from configure.ac.

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=106561


Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 0926f0af "meson,configure: include config.h automatically"