build w/-Wunused-parameter to match Android

The android build uses this option, so add it to local builds to match.

Change-Id: I4e478b7c294a90e7d52513e3c98499a76656297a
diff --git a/Makefile b/Makefile
index 47afdb7..43790a5 100644
--- a/Makefile
+++ b/Makefile
@@ -31,8 +31,9 @@
 CPPFLAGS += -DUSE_EXIT_ON_DIE
 endif
 
-CFLAGS += -Wextra -Wno-missing-field-initializers
-CXXFLAGS += -Wextra -Wno-missing-field-initializers
+MJ_COMMON_FLAGS = -Wunused-parameter -Wextra -Wno-missing-field-initializers
+CFLAGS += $(MJ_COMMON_FLAGS)
+CXXFLAGS += $(MJ_COMMON_FLAGS)
 
 USE_SYSTEM_GTEST ?= no
 ifeq ($(USE_SYSTEM_GTEST),no)