net/libsrtp2: Fix build with -fno-common

Add a patch from upstream that fixes the build of net/libsrtp2 with
-fno-common, which is the default with llvm 11.

MFH:		2020Q3 (implicit, -fno-common fixes, ok by joenum)

net/libsrtp2/files/patch-716a7386.c

@@ -0,0 +1,26 @@
+diff --git a/crypto/math/datatypes.c b/crypto/math/datatypes.c
+index 001584c1..4fcb3965 100644
+--- crypto/math/datatypes.c
++++ crypto/math/datatypes.c
+@@ -79,7 +79,7 @@ int octet_get_weight(uint8_t octet)
+ 
+ /* the value MAX_PRINT_STRING_LEN is defined in datatypes.h */
+ 
+-char bit_string[MAX_PRINT_STRING_LEN];
++static char bit_string[MAX_PRINT_STRING_LEN];
+ 
+ uint8_t srtp_nibble_to_hex_char(uint8_t nibble)
+ {
+diff --git a/test/util.c b/test/util.c
+index 2abc28e7..c0f76149 100644
+--- test/util.c
++++ test/util.c
+@@ -49,7 +49,7 @@
+ #include <stdint.h>
+ 
+ /* include space for null terminator */
+-char bit_string[MAX_PRINT_STRING_LEN + 1];
++static char bit_string[MAX_PRINT_STRING_LEN + 1];
+ 
+ static inline int hex_char_to_nibble(uint8_t c)
+ {