Fix namespacing of DynamicLib.cpp

`namespace dawn {` was written before some #includes

Bug: dawn:302
Change-Id: I1f0e507f63cc47b180d2021cf859d7964cfa4613
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/133622
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@google.com>
Kokoro: Austin Eng <enga@chromium.org>

src/dawn/common/DynamicLib.cpp

@@ -18,8 +18,6 @@
 
 #include "dawn/common/Platform.h"
 
-namespace dawn {
-
 #if DAWN_PLATFORM_IS(WINDOWS)
 #include "dawn/common/windows_with_undefs.h"
 #if DAWN_PLATFORM_IS(WINUWP)
@@ -31,6 +29,8 @@ namespace dawn {
 #error "Unsupported platform for DynamicLib"
 #endif
 
+namespace dawn {
+
 DynamicLib::~DynamicLib() {
     Close();
 }