cgen: fix warnings (#5820)

vlib/builtin/builtin_nix.c.v

@@ -94,7 +94,7 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 		beforeaddr := sframe.all_before('[')
 		cmd := 'addr2line -e $executable $addr'
 		// taken from os, to avoid depending on the os module inside builtin.v
-		f := C.popen(cmd.str, 'r')
+		f := C.popen(charptr(cmd.str), 'r')
 		if isnil(f) {
 			eprintln(sframe)
 			continue