Fixup "[analyzer] CStringChecker buffer access checks should check the first bytes"

0954dc3f broke a build bot:
https://lab.llvm.org/buildbot/#/builders/245/builds/13891

  clang/test/Analysis/string.c
  Line 74: incompatible redeclaration of library function 'memcpy'
  Line 74: 'memcpy' is a builtin with type 'void *(void *, const void *, unsigned int)'

Differential Revision: https://reviews.llvm.org/D159109