Fix a typo of nacl file name.

src/Makefile.am

@@ -44,7 +44,7 @@ nobase_include_HEADERS =                                        \
   google/protobuf/stubs/atomicops_internals_atomicword_compat.h \
   google/protobuf/stubs/atomicops_internals_macosx.h            \
   google/protobuf/stubs/atomicops_internals_mips_gcc.h          \
-  google/protobuf/stubs/atomicops_internals_pnal.h              \
+  google/protobuf/stubs/atomicops_internals_pnacl.h             \
   google/protobuf/stubs/atomicops_internals_x86_gcc.h           \
   google/protobuf/stubs/atomicops_internals_x86_msvc.h          \
   google/protobuf/stubs/common.h                                \