Mojo: Move public C system header files to mojo/public/c/system/.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'shared_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '..',
         ],
       },
       'sources': [
-        'public/system/async_waiter.h',
-        'public/system/core.h',
+        'public/c/system/async_waiter.h',
+        'public/c/system/core.h',
+        'public/c/system/macros.h',
+        'public/c/system/system_export.h',
         'public/system/core_cpp.h',
         'public/system/core_private.cc',
         'public/system/core_private.h',
-        'public/system/macros.h',
-        'public/system/system_export.h',
       ],
       'conditions': [
         ['OS=="mac"', {
           'xcode_settings': {
             # Make it a run-path dependent library.
             'DYLIB_INSTALL_NAME_BASE': '@rpath',
           },
           'direct_dependent_settings': {
             'xcode_settings': {
               # Look for run-path dependent libraries in the loader's directory.
@@ skipping 336 matching lines @@
         'mojo_bindings',
         'mojo_system',
       ],
       'export_dependent_settings': [
         'mojo_bindings',
         'mojo_system',
       ],
     },
   ],
 }