Move headers into structured include directory

So that meson can use that as an include path for dependencies.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>

Makefile.am

@@ -1,13 +1,13 @@
 gldir = $(includedir)/GL
 gl_HEADERS = \
-	glxint.h \
-	glxmd.h \
-	glxproto.h \
-	glxtokens.h
+	include/GL/glxint.h \
+	include/GL/glxmd.h \
+	include/GL/glxproto.h \
+	include/GL/glxtokens.h
 
 glinternaldir = $(includedir)/GL/internal
 glinternal_HEADERS = \
-	glcore.h
+	include/GL/internal/glcore.h
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = glproto.pc