[cmake] Include the new file in the CMake lists. Without this every

CMake build of any part of LLVM with LLDB checked out fails immediately.
=[

We appear to not even have a build bot covering the CMake build of LLDB
which makes this truly terrible. That needs to be fixed immediately.

llvm-svn: 218831

lldb/source/API/CMakeLists.txt

@@ -18,6 +18,7 @@ add_lldb_library(lldbAPI
   SBDeclaration.cpp
   SBError.cpp
   SBEvent.cpp
+  SBExecutionContext.cpp
   SBExpressionOptions.cpp
   SBFileSpec.cpp
   SBFileSpecList.cpp