build: fix compilation on another system

The <signal.h> header file is needed to define 'sig_atomic_t'.
(Don't know why on my main system it compiled fine without it.)

src/nano.c

@@ -33,7 +33,6 @@
 #include <langinfo.h>
 #endif
 #include <locale.h>
-#include <signal.h>
 #include <string.h>
 #ifdef HAVE_TERMIOS_H
 #include <termios.h>

src/nano.h

@@ -113,10 +113,11 @@
 #define N_(string) gettext_noop(string)
 		/* Mark a string that will be sent to gettext() later. */
 
-#include <stdlib.h>
-#include <sys/stat.h>
 #include <dirent.h>
 #include <regex.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <sys/stat.h>
 
 /* If we aren't using an ncurses with mouse support, exclude any
  * mouse routines, as they are useless then. */

src/text.c

@@ -27,7 +27,6 @@
 
 #include <errno.h>
 #include <fcntl.h>
-#include <signal.h>
 #include <string.h>
 #include <unistd.h>
 #include <sys/wait.h>