Add missing library dependency to fix build break in llvm-lto2

error message
CMakeFiles/llvm-lto2.dir/llvm-lto2.cpp.o: In function `dumpSymtab(int, char**)':
llvm-lto2.cpp:(.text._ZL10dumpSymtabiPPc+0x238): undefined reference to `llvm::getBitcodeFileContents(llvm::MemoryBufferRef)'
collect2: error: ld returned 1 exit status

llvm-svn: 306507