runtime(syntax): unlet b:filetype_in_cpp_family for cpp & squirrel

runtime(syntax): unlet b:filetype_in_cpp_family for cpp & squirrel

Update runtime/syntax/cpp.vim and runtime/syntax/squirrel.vim to unlet
b:filetype_in_cpp_family as it remains set even after updating the ft of
a file manually or through a modeline, not allowing c specific keywords
to be highlighted.

Since the variable b:filetype_in_cpp_family is only used by the c.vim
syntax script, unlet it directly after sourcing the c.vim runtime file
instead of at the end of the script.

Also update the last Change Header for both files.

closes: #13650

Signed-off-by: laburnumT <laburnumtec@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>