detect/pcre: fix compile warnings

detect-pcre.c:419:11: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  419 |     char *fcap = strstr(regexstr, "flow:");
      |           ^      ~~~~~~~~~~~~~~~~~~~~~~~~~
detect-pcre.c:420:11: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  420 |     char *pcap = strstr(regexstr, "pkt:");
      |           ^      ~~~~~~~~~~~~~~~~~~~~~~~~
detect-pcre.c:421:11: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  421 |     char *acap = strstr(regexstr, "alert:");
      |           ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~
3 errors generated.

src/detect-pcre.c

@@ -416,9 +416,9 @@ static DetectPcreData *DetectPcreParse (DetectEngineCtx *de_ctx,
     bool apply_match_limit = false;
 
     int cut_capture = 0;
-    char *fcap = strstr(regexstr, "flow:");
-    char *pcap = strstr(regexstr, "pkt:");
-    char *acap = strstr(regexstr, "alert:");
+    const char *fcap = strstr(regexstr, "flow:");
+    const char *pcap = strstr(regexstr, "pkt:");
+    const char *acap = strstr(regexstr, "alert:");
     /* take the size of the whole input as buffer size for the regex we will
      * extract below. Add 1 to please Coverity's alloc_strlen test. */
     size_t slen = strlen(regexstr) + 1;