qcacld-3.0: Fix the incremental compilation

BUILD_TAG changes in each build, which makes all files to recompile.
Notify build system that BULD_TAG is only used in wlan_hdd_main.c. This
reduces the recompile time significantly.

Change-Id: I40952e75eeb951ce8a693c8130dbc55a4a92eff5
CRs-Fixed: 2195100

Kbuild

@@ -2474,7 +2474,8 @@ CMN_IDS = $(shell cd "$(WLAN_COMMON_INC)" && \
 
 TIMESTAMP = $(shell date -u +'%Y-%m-%dT%H:%M:%SZ')
 BUILD_TAG = "$(TIMESTAMP); cld:$(CLD_IDS); cmn:$(CMN_IDS);"
-CDEFINES += -DBUILD_TAG=\"$(BUILD_TAG)\"
+# It's assumed that BUILD_TAG is used only in wlan_hdd_main.c
+CFLAGS_wlan_hdd_main.o += -DBUILD_TAG=\"$(BUILD_TAG)\"
 endif
 
 # Module information used by KBuild framework