Added missing "static" specifier found by gcc -Wtraditional.

This has somehow escaped from fbdaad9b0e7b.

src/os/unix/ngx_process.c

diff --git a/src/os/unix/ngx_process.c b/src/os/unix/ngx_process.c
index 24a63fb..2d37e21 100644
--- a/src/os/unix/ngx_process.c
+++ b/src/os/unix/ngx_process.c
@@ -306,7 +306,7 @@
 }
 
 
-void
+static void
 ngx_signal_handler(int signo)
 {
     char            *action;