Mark isHidlGralloc as 'const'

Bug: 128324105
Test: make
Change-Id: I06c5d830880711a015024e424e219e797fcb5a50
Signed-off-by: Roman Kiryanov <rkir@google.com>

system/gralloc/gralloc.cpp

@@ -67,9 +67,9 @@
 #define GOLDFISH_OFFSET_UNIT 8
 
 #ifdef GOLDFISH_HIDL_GRALLOC
-static bool isHidlGralloc = true;
+static const bool isHidlGralloc = true;
 #else
-static bool isHidlGralloc = false;
+static const bool isHidlGralloc = false;
 #endif
 
 int32_t* getOpenCountPtr(cb_handle_t* cb) {