Make aic7xxx -Werror conditional on make flag WARNINGS_BECOME_ERRORS

This should ensure it doesn't ordinarily break the builds, but will error out if the
builder requests it to.

drivers/scsi/aic7xxx/Makefile

@@ -33,7 +33,10 @@ aic79xx-y					+= aic79xx_osm.o	\
 						   aic79xx_proc.o	\
 						   aic79xx_osm_pci.o
 
-EXTRA_CFLAGS += -Idrivers/scsi -Werror
+EXTRA_CFLAGS += -Idrivers/scsi
+ifdef WARNINGS_BECOME_ERRORS
+EXTRA_CFLAGS += -Werror
+endif
 #EXTRA_CFLAGS += -g
 
 # Files generated that shall be removed upon make clean