systemtap: Add -lpthread option in LDFLAGS.

* CodeSourcery toolchain 2013.05-27 gives an error about
pthread_cacel symbol. Toolchain was not able to load
libpthread.so automatically so we have to link it manually.
* CB-2122

Signed-off-by: Noor Ahsan <noor_ahsan@mentor.com>

Backported from master branch.
Original commit is 296b92963f40fdd17b904f5ff3f6f780054ccd8f

Signed-off-by: Mikhail Durnev <mikhail_durnev@mentor.com>