Fix type of buffer to match zip_random signature.

Should fix a signedness warning on macOS, addressing
https://github.com/nih-at/libzip/pull/43/
diff --git a/regress/nonrandomopentest.c b/regress/nonrandomopentest.c
index 1da7497..b20a469 100644
--- a/regress/nonrandomopentest.c
+++ b/regress/nonrandomopentest.c
@@ -38,7 +38,7 @@
 
 int
 main(int argc, const char *argv[]) {
-    char buf[1024];
+    zip_uint8_t buf[1024];
     int i;
 
 #ifdef HAVE_CRYPTO