src: Explicitly include <netdb.h>.

cares_wrap.cc calls into functions like getnameinfo() and getaddrinfo().
These functions tend to be available implicitly through <uv.h>, but we'd
better still include this header explicitly.

On CloudABI, we make use of a custom implementation of libuv that does
not implicitly include header files like <netdb.h>.

src/cares_wrap.cc

@@ -38,6 +38,10 @@
 #include <vector>
 #include <unordered_set>
 
+#ifdef __POSIX__
+# include <netdb.h>
+#endif  // __POSIX__
+
 #if defined(__ANDROID__) || \
     defined(__MINGW32__) || \
     defined(__OpenBSD__) || \