interp: fix build error on Ubuntu 16.04
interp: fix build error on Ubuntu 16.04

"halbritt" reports that taskintf.cc has a problem building on Ubuntu 16.04,
possibly due to a glibc or libstdc++ standards-conformance bug.
    https://sourceware.org/bugzilla/show_bug.cgi?id=19439#c12

He tested and says that this change I proposed fixes it.

Signed-off-by: Jeff Epler <623a871f32baaca066adfe0cac08d7aa7d34027d@unpythonic.net>

ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror