add .brf for files generated by hyperref

.brf files are generated if the 'backref' or 'pagebackref' options of the hyperref package are enabled. These are intermediate files, so shouldn't be tracked.

LaTeX.gitignore

@@ -7,6 +7,7 @@
 *.blg
 *-blx.aux
 *-blx.bib
+*.brf
 *.dvi
 *.fdb_latexmk
 *.fls