For custom built upstream kernels on Debian, fix version detection

For custom built upstream kernels on Debian, fix version detection

Debian Bug: 856461

rrs@learner:~$ stap-prep
dpkg: warning: version '(gcc' has bad syntax: version number does not start with digit
Package linux-image-4.10.1+ version 4.10.1+-20 does not match version of currently running kernel: (gcc
 Consider apt-get upgrade && reboot
dpkg: warning: version '(gcc' has bad syntax: version number does not start with digit
Package linux-headers-4.10.1+ version 4.10.1+-20 does not match version of currently running kernel: (gcc
 Consider apt-get upgrade && reboot
You need package linux-image-4.10.1+-dbgsym but it does not seem to be available
 Debian -dbgsym packages are typically in a separate repository
 Follow https://wiki.debian.org/AutomaticDebugPackages to add this repository
2017-03-02 / 23:32:37 ♒♒♒  ☺

Signed-off-by: Ritesh Raj Sarraf <rrs@debian.org>