Fix typos, noticed by Clang.

llvm-svn: 113061
diff --git a/libcxx/src/stdexcept.cpp b/libcxx/src/stdexcept.cpp
index 5f3ffd7..316dff1 100644
--- a/libcxx/src/stdexcept.cpp
+++ b/libcxx/src/stdexcept.cpp
@@ -81,7 +81,7 @@
 
 }
 
-#pragma GCC visiblity pop
+#pragma GCC visibility pop
 
 namespace std  // purposefully not using versioning namespace
 {