Add the AL subdir to the list of include flags in the pkg-config file

This allows apps to include "al.h", "alc.h", etc, like with Creative's SDK and
CMake's FindOpenAL module, or as "AL/al.h" and "AL/alc.h" like before.

openal.pc.in

@@ -8,4 +8,4 @@ Description: OpenAL is a cross-platform 3D audio API
 Requires: @PKG_CONFIG_REQUIRES@
 Version: @PACKAGE_VERSION@
 Libs: -L${libdir} -l@LIBNAME@ @PKG_CONFIG_LIBS@
-Cflags: -I${includedir} @PKG_CONFIG_CFLAGS@
+Cflags: -I${includedir} -I${includedir}/AL @PKG_CONFIG_CFLAGS@