Add external/jsoncpp

external/jsoncpp is a new dependency of VNDK ABI tools.

Bug: 112877706
Test: OUT_DIR=out prebuilts/clang-tools/build-prebuilts.sh

default.xml

@@ -47,6 +47,7 @@
   <project path="external/clang" name="platform/external/clang" />
   <project path="external/compiler-rt" name="platform/external/compiler-rt" />
   <project path="external/googletest" name="platform/external/googletest" />
+  <project path="external/jsoncpp" name="platform/external/jsoncpp" />
   <project path="external/libcxx" name="platform/external/libcxx" />
   <project path="external/libcxxabi" name="platform/external/libcxxabi" />
   <project path="external/libunwind" name="platform/external/libunwind" />