Use strict function prototypes

Fixes clang error:
error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]

kitty/gl.c

@@ -39,7 +39,7 @@ check_for_gl_error(void UNUSED *ret, const char *name, GLADapiproc UNUSED funcpt
 }
 
 void
-gl_init() {
+gl_init(void) {
     static bool glad_loaded = false;
     if (!glad_loaded) {
         int gl_version = gladLoadGL(glfwGetProcAddress);