[make] remove the need for a module to manually add $(LOCAL_DIR)/include to the global include path

Pretty much every module in the system was already doing it, so do it
automatically.

target/zybo/rules.mk

@@ -10,9 +10,6 @@ ZYNQ_USE_SRAM ?= 1
 # we have sdram
 ZYNQ_SDRAM_SIZE := 0x20000000
 
-GLOBAL_INCLUDES += \
-	$(LOCAL_DIR)/include
-
 GLOBAL_DEFINES += \
 	EXTERNAL_CLOCK_FREQ=50000000 \
 	TARGET_HAS_DEBUG_LED=1