vre: Constify VRE_Capture

Spotted by Flexelint.

include/vre.h

@@ -60,8 +60,9 @@ vre_t *VRE_export(const vre_t *, size_t *);
 int VRE_error(struct vsb *, int err);
 int VRE_match(const vre_t *code, const char *subject, size_t length,
     int options, const volatile struct vre_limits *lim);
-int VRE_capture(vre_t *code, const char *subject, size_t length, int options,
-    txt *groups, size_t count, const volatile struct vre_limits *lim);
+int VRE_capture(const vre_t *code, const char *subject, size_t length,
+    int options, txt *groups, size_t count,
+    const volatile struct vre_limits *lim);
 int VRE_sub(const vre_t *code, const char *subject, const char *replacement,
     struct vsb *vsb, const volatile struct vre_limits *lim, int all);
 void VRE_free(vre_t **);

lib/libvarnish/vre.c

@@ -244,7 +244,7 @@ VRE_match(const vre_t *code, const char *subject, size_t length,
 }
 
 int
-VRE_capture(vre_t *code, const char *subject, size_t length, int options,
+VRE_capture(const vre_t *code, const char *subject, size_t length, int options,
     txt *groups, size_t count, const volatile struct vre_limits *lim)
 {
 	int i;