detect/app-layer-protocol: fix compile warning

detect-app-layer-protocol.c:160:11: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers
      [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  160 |     char *sep = strchr(arg, ',');
      |           ^     ~~~~~~~~~~~~~~~~
1 error generated.

(cherry picked from commit 3564f2f928)

src/detect-app-layer-protocol.c

@@ -157,7 +157,7 @@ static DetectAppLayerProtocolData *DetectAppLayerProtocolParse(const char *arg,
     AppProto alproto = ALPROTO_UNKNOWN;
 
     char alproto_copy[MAX_ALPROTO_NAME];
-    char *sep = strchr(arg, ',');
+    const char *sep = strchr(arg, ',');
     char *alproto_name;
     if (sep && sep - arg < MAX_ALPROTO_NAME) {
         strlcpy(alproto_copy, arg, sep - arg + 1);