tools/squashfs3-lzma: fix compilation with GCC15

Function pointers require proper types now.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18572
(cherry picked from commit d5c6452516)
Link: https://github.com/openwrt/openwrt/pull/18581
Signed-off-by: Robert Marko <robimarko@gmail.com>

tools/squashfs3-lzma/patches/160-gcc15.patch

@@ -0,0 +1,20 @@
+--- a/squashfs-tools/mksquashfs.c
++++ b/squashfs-tools/mksquashfs.c
+@@ -290,7 +290,7 @@ void restorefs()
+ }
+ 
+ 
+-void sighandler()
++void sighandler(int i)
+ {
+ 	if(interrupted == 1)
+ 		restorefs();
+@@ -302,7 +302,7 @@ void sighandler()
+ }
+ 
+ 
+-void sighandler2()
++void sighandler2(int i)
+ {
+ 	EXIT_MKSQUASHFS();
+ }