patch 8.2.1906: warning for signed/unsigned

Problem:    Warning for signed/unsigned.
Solution:   Use size_t instead of int. (Mike Williams)
diff --git a/src/usercmd.c b/src/usercmd.c
index ac01330..82b19bb 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -1269,10 +1269,10 @@
  * Generate text for the "cmod" command modifiers.
  * If "buf" is NULL just return the length.
  */
-    int
+    size_t
 produce_cmdmods(char_u *buf, cmdmod_T *cmod, int quote)
 {
-    int	    result = 0;
+    size_t  result = 0;
     int	    multi_mods = 0;
     int	    i;
     typedef struct {