gdb: make 'osv syms' work with httpserver.so in the image

We need to add mgmt/ to the search list. Otherwise it fails like this:

 gdb$ osv syms
 None 0x100000000000L Python Exception <type 'exceptions.TypeError'>
 cannot concatenate 'str' and 'NoneType' objects:
 Error occurred in Python command: cannot concatenate 'str' and
 'NoneType' objects

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>