fix(corstone-1000): fix Makefile error reporting

When trying to build for the Corstone-1000 platform without specifying a
valid TARGET_PLATFORM value, the "make" call reports a Makefile error
instead of the expected error messages pointing to the variable
omission:
====================
platform.mk: *** recipe commences before first target. Stop.
====================
This is due to the make's infamous special handling of the tab
character.

Fix the error report by replacing the tab with spaces.

Change-Id: I38264b6731793e5d5b929c189bb963e55bd5ce2d
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/arm/board/corstone1000/platform.mk

@@ -6,7 +6,7 @@
 
 # Making sure the corstone1000 platform type is specified
 ifeq ($(filter ${TARGET_PLATFORM}, fpga fvp),)
-	$(error TARGET_PLATFORM must be fpga or fvp)
+        $(error TARGET_PLATFORM must be fpga or fvp)
 endif
 
 CORSTONE1000_CPU_LIBS	+=lib/cpus/aarch64/cortex_a35.S