gralloc: Fix Compilation errors

Include -Wno-sign-conversion flag to avoid sign conversion warning.

Change-Id: I576ec57856ac1cab25ca657dcadbfbfa1dddf33c
diff --git a/common.mk b/common.mk
index 275c7d2..e362197 100644
--- a/common.mk
+++ b/common.mk
@@ -16,7 +16,7 @@
 
 #Common C flags
 common_flags := -DDEBUG_CALC_FPS -Wno-missing-field-initializers
-common_flags += -Wconversion -Wall -Werror
+common_flags += -Wconversion -Wall -Werror -Wno-sign-conversion
 ifeq ($(TARGET_IS_HEADLESS), true)
     common_flags += -DTARGET_HEADLESS
     LOCAL_CLANG := false