fdo#70393: move Mesa to a subdir of external

Change-Id: Ib3c0516297b0bd68b59c8cfc5c5488005c059c27
Reviewed-on: https://gerrit.libreoffice.org/6557Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

RepositoryExternal.mk

@@ -69,7 +69,7 @@ else
 
 define gb_LinkTarget__use_mesa_headers
 $(call gb_LinkTarget_set_include,$(1),\
-	-I$(SRCDIR)/Mesa/inc \
+	-I$(SRCDIR)/external/Mesa/inc \
 	$$(INCLUDE) \
 )