Add missing `void' keywords.

usr.sbin/pkg_install/add/main.c

@@ -344,7 +344,7 @@ getpackagesite(void)
 }
 
 static void
-usage()
+usage(void)
 {
     fprintf(stderr, "%s\n%s\n",
 	"usage: pkg_add [-viInfFrRMSK] [-t template] [-p prefix] [-P prefix] [-C chrootdir]",

usr.sbin/pkg_install/create/main.c

@@ -249,7 +249,7 @@ main(int argc, char **argv)
 }
 
 static void
-usage()
+usage(void)
 {
     fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n%s\n%s\n",
 "usage: pkg_create [-YNOhjnvyz] [-C conflicts] [-P pkgs] [-p prefix]",

usr.sbin/pkg_install/delete/main.c

@@ -170,7 +170,7 @@ main(int argc, char **argv)
 }
 
 static void
-usage()
+usage(void)
 {
     fprintf(stderr, "%s\n%s\n",
 	"usage: pkg_delete [-dDfGinrvxX] [-p prefix] pkg-name ...",

usr.sbin/pkg_install/info/main.c

@@ -282,7 +282,7 @@ main(int argc, char **argv)
 }
 
 static void
-usage()
+usage(void)
 {
     fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n",
 	"usage: pkg_info [-bcdDEfgGiIjkKLmopPqQrRsvVxX] [-e package] [-l prefix]",

usr.sbin/pkg_install/version/main.c

@@ -127,7 +127,7 @@ main(int argc, char **argv)
 }
 
 static void
-usage()
+usage(void)
 {
     fprintf(stderr, "%s\n%s\n%s\n",
 	"usage: pkg_version [-hIoqv] [-l limchar] [-L limchar] [[-X] -s string] [-O origin] [index]",