Arm: Fix error message printing in board makefile

Remove an incorrect tabulation in front of an $(error) function call
outside of a recipe, which caused the following text to be displayed:

  plat/arm/board/common/board_common.mk:36: *** recipe commences before first target.  Stop.

instead of:

  plat/arm/board/common/board_common.mk:36: *** "Unsupported ARM_ROTPK_LOCATION value".  Stop.

Change-Id: I8592948e7de8ab0c4abbc56eb65a53eb1875a83c
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

plat/arm/board/common/board_common.mk

@@ -33,7 +33,7 @@ $(BUILD_PLAT)/bl2/arm_dev_rotpk.o : $(ARM_ROTPK_HASH)
 $(warning Development keys support for FVP is deprecated. Use `regs` \
 option instead)
 else
-	$(error "Unsupported ARM_ROTPK_LOCATION value")
+$(error "Unsupported ARM_ROTPK_LOCATION value")
 endif
 
 $(eval $(call add_define,ARM_ROTPK_LOCATION_ID))