Switch soname from .so to .$(ext)

Reviewed By: dinhviethoa

Differential Revision: D5595092

fbshipit-source-id: f6fe1822938693d59f884f0957c7b73d50977668

ReactCommon/cxxreact/BUCK

@@ -62,7 +62,7 @@ rn_xplat_cxx_library(
         "-fno-omit-frame-pointer",
         "-fexceptions",
     ],
-    soname = "libxplat_react_module_samplemodule.so",
+    soname = "libxplat_react_module_samplemodule.$(ext)",
     visibility = [
         "PUBLIC",
     ],