Change the guard for bootblock_systemagent_early_init to ENV_BOOTBLOCK

The definition of bootblock_systemagent_early_init was guarded by
!ENV_RAMSTAGE. But it's only called in the bootblock. So guard it
with ENV_BOOTBLOCK instead.

Change-Id: I143cf72e4a63b176e4772575e7a60a2a611e4ad9
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32697
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lijian Zhao <lijian.zhao@intel.com>
Reviewed-by: Subrata Banik <subrata.banik@intel.com>

src/soc/intel/common/block/systemagent/systemagent_early.c

@@ -24,7 +24,7 @@
 
 #include "systemagent_def.h"
 
-#if !ENV_RAMSTAGE
+#if ENV_BOOTBLOCK
 void bootblock_systemagent_early_init(void)
 {
 	uint32_t reg;