Add libevent to manifest

Bug: 22230368

default.xml

@@ -174,6 +174,7 @@
   <project path="external/libcxx" name="platform/external/libcxx" groups="pdk" />
   <project path="external/libcxxabi" name="platform/external/libcxxabi" groups="pdk" />
   <project path="external/libedit" name="platform/external/libedit" />
+  <project path="external/libevent" name="platform/external/libevent" />
   <project path="external/libexif" name="platform/external/libexif" groups="pdk-cw-fs" />
   <project path="external/libhevc" name="platform/external/libhevc" />
   <project path="external/libgsm" name="platform/external/libgsm" groups="pdk" />