Use more specific hasGlobalName() for stdlib function free(3)

Based on the CodeQL documentation's example of strncpy(3) and strlen(3): https://codeql.github.com/docs/codeql-language-guides/hash-consing-and-value-numbering/#example-query

Author: mario-campos

cpp/ql/src/experimental/Best Practices/GuardedFree.ql

@@ -14,7 +14,7 @@
 import cpp
 
 class FreeCall extends FunctionCall {
-  FreeCall() { this.getTarget().hasName("free") }
+  FreeCall() { this.getTarget().hasGlobalName("free") }
 }
 
 from IfStmt stmt, FreeCall fc, Variable v