Fix build warning

Commit c543f59696 introduced the following warnings:

objects/screen.c:307:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
objects/screen.c:307:1: warning: no previous prototype for ‘screen_scan_randr_monitors’ [-Wmissing-prototypes]

Signed-off-by: Uli Schlachter <psychon@znc.in>

objects/screen.c

@@ -304,6 +304,7 @@ screen_scan_randr_monitors(void)
     return found;
 }
 #else
+static bool
 screen_scan_randr_monitors(void)
 {
     return false;