Add/Update include guards to headers

[kala-ghoda]

• Add include guards for header.h, npb-C.h, npbparams.h
• Update include guards for MemRegion.h, PTACallGraph.h

[kala-ghoda]

Hi @tommymcm, thanks for assigning the task to me. I have created an initial PR just for ".h" header files. If this is according to the standards, I will proceed with ".hpp" headers. Thanks! 😄

[tommymcm]

Hey @kala-ghoda, the format of this is correct, but the include guards are not needed for any files under the test/ directory, only those under external/ (which are good the way you have them) and src/.

Thanks! Tommy

[kala-ghoda]

Thanks, pushing fresh commit.