Fix extern symbol declarations to be consistent with the definition.

Bug: https://bugs.gentoo.org/941584
Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>

diff -rup a/src/apcupsd.c a/src/apcupsd.c
--- a/src/apcupsd.c
+++ b/src/apcupsd.c
@@ -31,7 +31,7 @@ static void daemon_start(void);
 int pidcreated = 0;
 extern int kill_on_powerfail;
 extern FILE *trace_fd;
-extern char *pidfile;
+extern const char *pidfile;
 
 /*
  * The terminate function and trapping signals allows apcupsd
diff -rup a/src/options.c b/src/options.c
--- a/src/options.c
+++ b/src/options.c
@@ -62,7 +62,7 @@ int terminate_on_powerfail = FALSE;
 int kill_on_powerfail = FALSE;
 int dumb_mode_test = FALSE;        /* for testing dumb mode */
 int go_background = TRUE;
-extern char *pidfile;
+extern const char *pidfile;
 extern bool trace;
 
 static void print_usage(char *argv[])