Define '_GNU_SOURCE' when compiling on Linux
Define '_GNU_SOURCE' when compiling on Linux

Adding '_GNU_SOURCE' to 'DEFINES' in Makefile removes the following
warning:

  builtin-import.c: In function ‘pstore_table__import_columns’:
  builtin-import.c:146: warning: implicit declaration of function ‘strndup’
  builtin-import.c:146: warning: incompatible implicit declaration of built-in function ‘strndup’

Signed-off-by: Karim Osman <94396faf6cc817be76165477038b3cfa832e89a4@reaktor.fi>
Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@kernel.org>

penberg/pstore,penberg/pstore,penberg/pstore,penberg/pstore