Remove `volatile` from the `__errno` declaration.

Remove `volatile` from the `__errno` declaration.

This breaks code that declares `errno` itself for whatever crazy reason:

  b.c:22:12: error: conflicting types for '__errno'
  extern int errno;
             ^
  /usr/local/google/ndkports/toolchain/bin/../sysroot/usr/include/errno.h:47:20: note: expanded from macro 'errno'
  #define  errno   (*__errno())
                     ^
  /usr/local/google/ndkports/toolchain/bin/../sysroot/usr/include/errno.h:44:15: note: previous declaration is here
  volatile int* __errno(void) __attribute_const__;
                ^

Bug: N/A
Test: built various bits of GNU source
Change-Id: I27c03bf3bde419a001f98f1ea6c267c847f31271