sendmail: fix compilation without sys/cdefs

sys/cdefs is deprecated. It's also not included with musl.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

mail/sendmail/patches/210-cdefs.patch

@@ -0,0 +1,10 @@
+--- a/include/sm/os/sm_os_linux.h
++++ b/include/sm/os/sm_os_linux.h
+@@ -33,7 +33,6 @@
+ # endif /* LINUX_VERSION_CODE */
+ #endif /* SM_CONF_SHM */
+ 
+-#define SM_CONF_SYS_CDEFS_H	1
+ #ifndef SM_CONF_SEM
+ # define SM_CONF_SEM	2
+ #endif /* SM_CONF_SEM */