Fix -Wdeprecated-non-prototype warnings

[Matthew Smith]

Two K&R function definitions in vars.c and a function pointer with an empty argument list in traceroot.c cause warnings in clang 15, which may become errors by default in clang 16.

vars.c:94:13: error: a function definition without a prototype is deprecated in all versions of C and is not supported in C2x [-Werror,-Wdeprecated-non-prototype]
struct var *lookup_by_name(name)
            ^
vars.c:105:13: error: a function definition without a prototype is deprecated in all versions of C and is not supported in C2x [-Werror,-Wdeprecated-non-prototype]
struct var *lookup_by_obj(obj)
            ^
traceroot.c:257:36: error: passing arguments to a function without a prototype is deprecated in all versions of C and is not supported in C2x [-Werror,-Wdeprecated-non-prototype]
                    context_scanner(compare_pointer, c);
                                    ^

[Douglas Katzman (dougk)]

TBH we got 1 warning less without this patch. Now we get:

traceroot.c: In function 'deduce_thread':
traceroot.c:258:37: warning: passing argument 1 of 'context_scanner' from incompatible pointer type [-Wincompatible-pointer-types]
  258 | context_scanner(compare_pointer, c);
      | ^~~~~~~~~~~~~~~
      | |
      | void (*)(void *)
traceroot.c:258:37: note: expected 'void (*)(void)' but argument is of type 'void (*)(void *)'

Can you provide a better fix please?

[Douglas Katzman (dougk)]

was not released in full. It caused other warnings, so I backed out part of it