maint: pr: avoid new GCC 7 warning

* src/pr.c (main): Avoid this warning from GCC 7:
src/pr.c:1119:6: error: assuming signed overflow does not occur when \
  simplifying conditional to constant [-Werror=strict-overflow]
   if (n_files == 0)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/pr.c b/src/pr.c
index d4549a3bb..a22d550a7 100644
--- a/src/pr.c
+++ b/src/pr.c
@@ -847,7 +847,7 @@ separator_string (const char *optarg_S)
 int
 main (int argc, char **argv)
 {
-  int n_files;
+  unsigned int n_files;
   bool old_options = false;
   bool old_w = false;
   bool old_s = false;