gpg4libre: include also plain C headers from internal gpgme

Change-Id: Iddae7a3e664273b53a49450527f55814a9ef6f55
Reviewed-on: https://gerrit.libreoffice.org/34610Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

RepositoryExternal.mk

@@ -3427,6 +3427,7 @@ endef
 define gb_LinkTarget__use_gpgmepp
 $(call gb_LinkTarget_set_include,$(1),\
 	-I$(call gb_UnpackedTarball_get_dir,gpgme)/lang/cpp/src \
+	-I$(call gb_UnpackedTarball_get_dir,gpgme)/src \
 	$$(INCLUDE) \
 )
 $(call gb_LinkTarget_add_libs,$(1),\