gitignore: add doc/*.tag.prep

Ignore files created during the two-pass doxygen generation that was
introduced in Ib03d0b70d536c8f1386def666c89106a840f7363.

Change-Id: I719bbc968420c462426d2c0ce703c7f3b2c1139e
diff --git a/.gitignore b/.gitignore
index 451c883..51c443f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -81,6 +81,7 @@
 doc/gb
 doc/html.tar
 doc/*.tag
+doc/*.tag.prep
 tags
 
 src/crc*gen.c