[gdb/testsuite] Fix untested in gdb.base/frame-view.exp

When running test-case gdb.base/frame-view.exp, I see:
...
gdb compile failed, ld: frame-view0.o: in function `main':
frame-view.c:73: undefined reference to `pthread_create'
ld: frame-view.c:76: undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
UNTESTED: gdb.base/frame-view.exp: failed to prepare
...

Fix this by adding pthreads to the compilation flags.

Tested on x86_64-linux.