proto_msrp: Fix gcc-10 false positive on ubuntu-20.04

In function ‘memcpy’, inlined from ‘_ident_builder.constprop’
at msrp_signaling.c:1198:3:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34:10: error:
‘__builtin___memcpy_chk’ writing 32 bytes into a region of size 18
overflows the destination [-Werror=stringop-overflow=]

   34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(cherry picked from commit da3782e)

modules/proto_msrp/msrp_signaling.c

@@ -1158,7 +1158,7 @@ static char * _ident_builder( unsigned short hash, unsigned short idx,
 {
 	#define IDENT_BUF_MAX_LEN 20
 	/* format is hash_hexa.idx_hexa.rand_hexa.padding , max 20 chars*/
-	static char ident_s[IDENT_BUF_MAX_LEN + 1];
+	static char ident_s[IDENT_BUF_MAX_LEN + 1 + MD5_LEN /*fix gcc warn*/];
 	unsigned short rnd;
 	int size;
 	char *p;