agp: Export intel_gtt_read_pte, required by i915kms

This isn't used by modern cards, but is needed for i915kms to load
on a system that has agp as a module not compiled into the kernel.

PR:		291214
Reviewed by:	cy
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53906

sys/modules/agp/Makefile

@@ -36,6 +36,7 @@ EXPORT_SYMS+=	intel_gtt_clear_range	\
 		intel_gtt_install_pte	\
 		intel_gtt_get		\
 		intel_gtt_chipset_flush	\
+		intel_gtt_read_pte	\
 		intel_gtt_unmap_memory	\
 		intel_gtt_map_memory	\
 		intel_gtt_insert_sg_entries \