multimedia/ringrtc: Fix build on recent main

After src snapshot 0c47b9c211e, the build began failing with:

../../third_party/nasm/include/compiler.h:249:21: error: static
  declaration of 'mempcpy' follows non-static declaration
  249 | static inline void *mempcpy(void *dst, const void *src, size_t n)
      |                     ^
/usr/include/string.h:71:8: note: previous declaration is here
   71 | void    *(mempcpy)(void * __restrict,
                  const void * __restrict, size_t);

Fix the build by restoring a default definition of HAVE_MEMPCPY since
we have had mempcpy() since 13.1.

Approved by:	kevans, mikael (maintainer)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D46053

multimedia/ringrtc/files/patch-third__party_nasm_config_config-linux.h

@@ -1,4 +1,4 @@
---- src/webrtc/src/third_party/nasm/config/config-linux.h	2021-03-15 17:51:55 UTC
+--- src/webrtc/src/third_party/nasm/config/config-linux.h.orig	2024-03-14 18:07:21 UTC
 +++ src/webrtc/src/third_party/nasm/config/config-linux.h
 @@ -160,7 +160,7 @@
  
@@ -18,7 +18,7 @@
  
  /* Define to 1 if you have the `faccessat' function. */
  #define HAVE_FACCESSAT 1
-@@ -327,16 +327,16 @@
+@@ -327,10 +327,10 @@
  #define HAVE_ISASCII 1
  
  /* Define to 1 if you have the `iscntrl' function. */
@@ -31,13 +31,6 @@
  
  /* Define to 1 if you have the <memory.h> header file. */
  #define HAVE_MEMORY_H 1
- 
- /* Define to 1 if you have the `mempcpy' function. */
--#define HAVE_MEMPCPY 1
-+/* #undef HAVE_MEMPCPY */
- 
- /* Define to 1 if you have a working `mmap' system call. */
- #define HAVE_MMAP 1
 @@ -357,7 +357,7 @@
  #define HAVE_STDARG_H 1