Add prebuilt/gdb/{darwin,linux}-x86 to manifest.

Bug: http://b/21920612
Change-Id: Id1a2927a300cac5dd0bafcb7e53c2b8895f2fc06

default.xml

@@ -480,6 +480,8 @@
   <project path="prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8" name="platform/prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8" groups="pdk-fs" />
   <project path="prebuilts/gcc/linux-x86/mips/mips64el-linux-android-4.9" name="platform/prebuilts/gcc/linux-x86/mips/mips64el-linux-android-4.9" groups="pdk,linux,mips" />
   <project path="prebuilts/gcc/linux-x86/x86/x86_64-linux-android-4.9" name="platform/prebuilts/gcc/linux-x86/x86/x86_64-linux-android-4.9" groups="pdk,linux,x86" />
+  <project path="prebuilts/gdb/darwin-x86" name="platform/prebuilts/gdb/darwin-x86" groups="darwin" />
+  <project path="prebuilts/gdb/linux-x86" name="platform/prebuilts/gdb/linux-x86" groups="linux" />
   <project path="prebuilts/go/darwin-x86" name="platform/prebuilts/go/darwin-x86" groups="darwin,tradefed" />
   <project path="prebuilts/go/linux-x86" name="platform/prebuilts/go/linux-x86" groups="linux,tradefed" />
   <project path="prebuilts/gradle-plugin" name="platform/prebuilts/gradle-plugin" groups="pdk-cw-fs,pdk-fs" />