Use -fvisibility=hidden to build libc_gdtoa.

Bug: 12177745
Change-Id: Ia4ac71e1a3a237e764c809cc591622f8952ed642
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index 2c797c5..58538ff 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -65,9 +65,10 @@
 extern int posix_memalign(void **memptr, size_t alignment, size_t size);
 
 extern double atof(const char*);
-extern double strtod(const char*, char**);
-extern float strtof(const char*, char**);
-extern long double strtold(const char*, char**);
+
+extern double strtod(const char*, char**) __LIBC_ABI_PUBLIC__;
+extern float strtof(const char*, char**) __LIBC_ABI_PUBLIC__;
+extern long double strtold(const char*, char**) __LIBC_ABI_PUBLIC__;
 
 extern int atoi(const char*) __purefunc;
 extern long atol(const char*) __purefunc;