[binutils-gdb] Remove EXTERN from buildsym.h

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT b80a981d0867f554d382eef9fc16081b12ac0543 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: b80a981d0867f554d382eef9fc16081b12ac0543

Remove EXTERN from buildsym.h

Nothing in buildsym.h relies on the "EXTERN" method of
declaration/definition, so remove the traces.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* buildsym.h (EXTERN): Don't define or undef.
	* buildsym.c (EXTERN): Don't define.