gprofng: Use $(DESTDIR) in install-examples

Use $(DESTDIR) in install-examples to fix

mkdir -p -- /usr/share/doc//gprofng
mkdir: cannot create directory ‘/usr/share/doc//gprofng’: Permission denied

for "make install DESTDIR=...".

	* doc/Makefile.am (install-examples): Use $(DESTDIR).
	* doc/Makefile.in: Regenerated.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>