gdb_server: fix semicolon that depends on #define

supported_features_cmd is a string which part of is added based on a

However, the semicolon to do so was being only included if the #define
was there; if not it was generating a syntax error. Place it out of
the #ifdef block.

Change-Id: Idd7e80519719b5ad432971e758a05546df8a89e2
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>