runtests.pl: Fix perl call to include srcdir

- Use explicit include opt for perl calls.

Prior to this change some scripts couldn't find their dependencies.

At the top, perl is called using with the "-Isrcdir" option, and it
works:

https://github.com/curl/curl/blob/curl-7_63_0/tests/runtests.pl#L183

But on line 3868, that option is omitted. This caused problems for me,
as the symbol-scan.pl script in particular couldn't find its
dependencies properly:

https://github.com/curl/curl/blob/curl-7_63_0/tests/runtests.pl#L3868

This patch fixes that oversight by making calls to perl sub-shells
uniform.

Closes https://github.com/curl/curl/pull/3496