configure.ac: fix the detection of expat with pkg-config

The pkg-config module was called "EXPAT" instead of "expat" in
PKG_CHECK_EXISTS. This seems to have been wrong because the wrong
argument was copied from PKG_CHECK_MODULES.

Cc: "10.0" "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
1 file changed