Add explicit -Werror=unused-variable

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Eshan Kelkar <eshankelkar@galorithm.com>
(cherry picked from commit 8295945011025305c97de879173fb9f239d4bb79)

Author: Jakuje

CompilerChecks.cmake

@@ -48,6 +48,7 @@ if (UNIX)
     add_c_compiler_flag("-Werror=implicit-int" SUPPORTED_COMPILER_FLAGS)
     add_c_compiler_flag("-Wint-conversion" SUPPORTED_COMPILER_FLAGS)
     add_c_compiler_flag("-Werror=int-conversion" SUPPORTED_COMPILER_FLAGS)
+    add_c_compiler_flag("-Werror=unused-variable" SUPPORTED_COMPILER_FLAGS)
 
     check_c_compiler_flag("-Wformat" REQUIRED_FLAGS_WFORMAT)
     if (REQUIRED_FLAGS_WFORMAT)