Bug#957812: slirp: ftbfs with GCC-10

[Bernhard Übelacker]

Dear Maintainer,
the patch "949003_explicit_extern_declaration.patch" attached in [1],
should help against the exact issue this bug is about.

Kind regards,
Bernhard

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=949003#35