bionic: stubs: Make internal symbol static

Thread-specific state for the stubs functions should not be exposed to
applications.

Change-Id: I4d35dab6009dab8db7781671ac5cc9b5f6904e84
diff --git a/libc/bionic/stubs.c b/libc/bionic/stubs.c
index d495674..a01d64c 100644
--- a/libc/bionic/stubs.c
+++ b/libc/bionic/stubs.c
@@ -40,8 +40,8 @@
 /** Thread-specific state for the stubs functions
  **/
 
-pthread_once_t   the_once = PTHREAD_ONCE_INIT;
-pthread_key_t    the_key;
+static pthread_once_t   the_once = PTHREAD_ONCE_INIT;
+static pthread_key_t    the_key;
 
 typedef struct {
     struct passwd  passwd;