scylla-gdb.py: Fix lookup of global symbols
scylla-gdb.py: Fix lookup of global symbols

Fixes errors like the one below:

  (gdb) scylla memory
  Python Exception <class 'gdb.error'> A syntax error in expression, near `memory::cpu_mem'.:
  Error occurred in Python command: A syntax error in expression, near `memory::cpu_mem'.

Wrapping the symbol in quotes instructs GDB to lookup in the global
context instead of the context of current frame.
Message-Id: <60d42b543786a2cd4c0c04770694861f74e271c4@scylladb.com>

scylladb/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla