all: fix a compiler warning about function declarations

warning: function declaration isn’t a prototype [-Wstrict-prototypes]

In C function() and function(void) are two different prototypes (as opposed to
C++).
function()     accepts an arbitrary number of arguments
function(void) accepts zero arguments

src/main.c

@@ -160,7 +160,7 @@ parse_state_file (const char *filename,
 }
 
 static void
-_set_g_fatal_warnings ()
+_set_g_fatal_warnings (void)
 {
 	GLogLevelFlags fatal_mask;