builtin-extend: Add missing include
builtin-extend: Add missing include

Fixes the following build breakage on Linux:

  CC       builtin-extend.o
builtin-extend.c: In function ‘cmd_extend’:
builtin-extend.c:181: error: ‘errno’ undeclared (first use in this function)
builtin-extend.c:181: error: (Each undeclared identifier is reported only once
builtin-extend.c:181: error: for each function it appears in.)

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@kernel.org>

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