Add external/Microsoft-GSL.

Test: repo sync external/Microsoft-GSL
Bug: http://b/24296825

default.xml

@@ -231,6 +231,7 @@
   <project path="external/mdnsresponder" name="platform/external/mdnsresponder" groups="pdk" />
   <project path="external/mesa3d" name="platform/external/mesa3d" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/messageformat" name="platform/external/messageformat" groups="pdk" />
+  <project path="external/Microsoft-GSL" name="platform/external/Microsoft-GSL" groups="pdk" />
   <project path="external/minijail" name="platform/external/minijail" groups="pdk" />
   <project path="external/mksh" name="platform/external/mksh" groups="pdk" />
   <project path="external/mmc-utils" name="platform/external/mmc-utils" groups="pdk" />