patch 9.0.1544: recent glibc marks sigset() as a deprecated

Problem:    Recent glibc marks sigset() as a deprecated.
Solution:   Use sigaction() in mch_signal() if possible. (Ozaki Kiichi,
            closes #12373)

src/misc1.c

@@ -2208,7 +2208,7 @@ prepare_to_exit(void)
     // Ignore SIGHUP, because a dropped connection causes a read error, which
     // makes Vim exit and then handling SIGHUP causes various reentrance
     // problems.
-    signal(SIGHUP, SIG_IGN);
+    mch_signal(SIGHUP, SIG_IGN);
 #endif
 
 #ifdef FEAT_GUI