ACPI GNVS: Replace uses of smm_get_gnvs()

Change-Id: I7b657750b10f98524f011f5254e533217fe94fd8
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42849
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/include/cpu/x86/smm.h

@@ -86,7 +86,9 @@ struct smm_module_params {
 typedef asmlinkage void (*smm_handler_t)(void *);
 
 /* SMM Runtime helpers. */
-struct global_nvs *smm_get_gnvs(void);
+#if ENV_SMM
+extern struct global_nvs *gnvs;
+#endif
 
 /* Entry point for SMM modules. */
 asmlinkage void smm_handler_start(void *params);