tools: m4: update to 1.4.21

Changelog: https://github.com/autotools-mirror/m4/blob/branch-1.4/NEWS

This update fixes a build error on my system:
./string.h:777:20: error: expected identifier or '(' before '_Generic'
  777 | _GL_EXTERN_C void *memchr (const void *__s, int __c, size_t __n)
      |                    ^~~~~~

Signed-off-by: John Audia <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21987
Signed-off-by: Robert Marko <[email protected]>

tools/m4/Makefile

@@ -8,11 +8,11 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=m4
 PKG_CPE_ID:=cpe:/a:gnu:m4
-PKG_VERSION:=1.4.20
+PKG_VERSION:=1.4.21
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=@GNU/$(PKG_NAME)
-PKG_HASH:=6ac4fc31ce440debe63987c2ebbf9d7b6634e67a7c3279257dc7361de8bdb3ef
+PKG_HASH:=38ae59f7a30bf9c108193cc5c25fbb06014f21e230c7ede2eff614f7b7c37ed8
 
 HOST_BUILD_PARALLEL:=1