[clang-doc] Fix build break with BUILD_SHARED_LIBS=ON

The error message:

/usr/bin/ld: CMakeFiles/clang-doc.dir/ClangDocMain.cpp.o: undefined refe
rence to symbol '_Z20getMustacheHtmlFilesN4llvm9StringRefERN5clang3doc15
ClangDocContextE' /usr/bin/ld: /work/kparzysz/git/llvm.org/b/x86/./lib/l
ibclangDocSupport.so.21.0git: error adding symbols: DSO missing from com
mand line
collect2: error: ld returned 1 exit status
make[2]: *** [tools/clang/tools/extra/clang-doc/tool/CMakeFiles/clang-do
c.dir/build.make:107: bin/clang-doc] Error 1

Author: kparzysz

clang-tools-extra/clang-doc/tool/CMakeLists.txt

@@ -9,6 +9,7 @@ clang_target_link_libraries(clang-doc
   clangAST
   clangASTMatchers
   clangBasic
+  clangDocSupport
   clangFrontend
   clangTooling
   clangToolingCore