remove uv.h dependency from header files

src/util/uv.cpp

@@ -0,0 +1,19 @@
+#include <mbgl/util/uv.hpp>
+
+#include <uv.h>
+
+namespace uv {
+
+std::string cwd() {
+    size_t max = 0;
+    std::string dir;
+    do {
+        max += 256;
+        dir.resize(max);
+        uv_cwd(const_cast<char *>(dir.data()), &max);
+    } while (max == dir.size());
+    dir.resize(max - 1);
+    return dir;
+}
+
+}