rtos: Fix wrong allocation in linux_get_symbol_list_to_lookup

rtos: Fix wrong allocation in linux_get_symbol_list_to_lookup

linux_get_symbol_list_to_lookup allocates to few memory. On 64 bit
systems the error did not show due to char* being twice its size,
leaving accidentally enough space.

This patch makes linux_get_symbol_list_to_lookup behave identical
to all other RTOS.

Change-Id: I290ea241fb20b65585c8be14609a92fdbd2a307d
Signed-off-by: Matthias Blaicher <matthias@blaicher.com>
Reviewed-on: http://openocd.zylin.com/934
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>