Fix header identification

Hi hackers,

PFA a patch that fixes up the identification for 4 header files.

I did a little archaeology trying to find plausible reasons for why we
committed the wrong identification in the first place, and here's what I
came up with:

jsonfuncs.h was created in ce0425b162d0a to house backend-only json
function declarations previously in jsonapi.h, and the identification
was a copy-pasta from jsonapi.h (then in src/include/utils).

jsonapi.h was wholesale moved to common in beb4699091e9f without
changing identification.

partdesc.h was created in 1bb5e78218107 but I can't find a good excuse
why we made a mistake then, except for (maybe) the prototype for
RelationBuildPartitionDesc() was moved from partcache.h and so we may
have also taken its identification line, although that sounds arbitrary.

llvmjit_emit.h was created with the wrong identification, probably due
to a personal template...

Cheers,
Jesse