process: SIGCLD is not portable

MUSL libc doesn't have it, for instance.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/process.cc

@@ -191,7 +191,9 @@ namespace crucible {
 #endif
 		NTOA_TABLE_ENTRY_ENUM(SIGSTKFLT),
 		NTOA_TABLE_ENTRY_ENUM(SIGIO),
+#ifdef SIGCLD
 		NTOA_TABLE_ENTRY_ENUM(SIGCLD),
+#endif
 		NTOA_TABLE_ENTRY_ENUM(SIGPWR),
 #ifdef SIGINFO
 		NTOA_TABLE_ENTRY_ENUM(SIGINFO),