CLEANUP: Add missing header to hlua_fcn.c

Found with -Wmissing-prototypes:

    src/hlua_fcn.c:53:5: fatal error: no previous prototype for function 'hlua_checkboolean' [-Wmissing-prototypes]
    int hlua_checkboolean(lua_State *L, int index)
        ^
    src/hlua_fcn.c:53:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
    int hlua_checkboolean(lua_State *L, int index)
    ^
    static
    1 error generated.

src/hlua_fcn.c

@@ -26,6 +26,7 @@
 #include <haproxy/cli-t.h>
 #include <haproxy/errors.h>
 #include <haproxy/hlua-t.h>
+#include <haproxy/hlua_fcn.h>
 #include <haproxy/http.h>
 #include <haproxy/net_helper.h>
 #include <haproxy/pattern-t.h>