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

0954dc3fb9214b994623f5306473de075f8e3593 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

clang/test/Analysis/string.c

@@ -71,7 +71,7 @@ void clang_analyzer_eval(int);
 int scanf(const char *restrict format, ...);
 void *malloc(size_t);
 void free(void *);
-void *memcpy(void *dest, const void *src, unsigned long n);
+void *memcpy(void *dest, const void *src, size_t n);
 
 //===----------------------------------------------------------------------===
 // strlen()