libc: Add SIGINT define

libs/libc/include/signal.h

@@ -1,7 +1,9 @@
 #ifndef _SIGNAL_H
 #define _SIGNAL_H
 
-typedef int sig_atomic_t; // FIXME: Implement signals (I'm trying to build bc, this header is only to satisfy it) and
+typedef int sig_atomic_t; // On the x86, writes to aligned 32-bit and 64-bit integers are always atomic. (Or that's what
-                          // use a proper atomic integer type.
+                          // I understood)
+
+#define SIGINT 1 // Not implemented.
 
 #endif