BPF: fix libLLVMBPFCodeGen.so build failure

Buildbot reported a build failure when building shared
library libLLVMBPFCodeGen.so with unknown reference to
"createCFGSimplificationPass".

Commit 87cba434 ("BPF: add a SimplifyCFG IR pass during
generic Scalar/IPO optimization") added an IR pass SimplifyCFG
by BPF target. The commit called function
createCFGSimplificationPass() defined in "Scalar" library.
Add this library in Target/BPF/LLVMBuild.txt so
shared library build can succeed.