Fix build for some non-x86 archs

Ref:
https://buildd.debian.org/status/fetch.php?pkg=finit&arch=mips64el&ver=3.2%7Erc3-2&stamp=1609910244&raw=0
https://buildd.debian.org/status/fetch.php?pkg=finit&arch=sparc64&ver=3.2%7Erc3-2&stamp=1609869348&raw=0

src/sig.c

@@ -138,7 +138,9 @@ static struct sigmap {
 	/* Nonstandard signals found in all modern POSIX systems
 	   (including both BSD and Linux).  */
 	{ SIGWINCH,  "SIGWINCH"  },
+#ifdef SIGSTKFLT
 	{ SIGSTKFLT, "SIGSTKFLT" },
+#endif
 	{ SIGPWR,    "SIGPWR"    },
 
 	/* Archaic names for compatibility.  */