strerror: Fix compiler warning "empty expression"

- Remove the final semi-colon in the SEC2TXT() macro definition.

Before:  #define SEC2TXT(sec) case sec: txt = #sec; break;

After:   #define SEC2TXT(sec) case sec: txt = #sec; break

Prior to this change SEC2TXT(foo); would generate break;; which caused
the empty expression warning.

Ref: https://github.com/curl/curl/commit/5b22e1a#r36458547

lib/strerror.c

@@ -872,7 +872,7 @@ const char *Curl_sspi_strerror(int err, char *buf, size_t buflen)
     case SEC_E_OK:
       txt = "No error";
       break;
-#define SEC2TXT(sec) case sec: txt = #sec; break;
+#define SEC2TXT(sec) case sec: txt = #sec; break
     SEC2TXT(CRYPT_E_REVOKED);
     SEC2TXT(SEC_E_ALGORITHM_MISMATCH);
     SEC2TXT(SEC_E_BAD_BINDINGS);