.gitignore: Add Doxygen dir `libflashrom-doc/`

Change-Id: Id25d05cdf6107cc7a99b94a8523e23bd8698c2d6
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/20811
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
diff --git a/.gitignore b/.gitignore
index 19a4365..2d621dc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,6 +8,7 @@
 /flashrom-*
 /flashrom.exe
 /flashrom.8
+/libflashrom-doc/
 
 /util/ich_descriptors_tool/ich_descriptors_tool
 /util/ich_descriptors_tool/ich_descriptors_tool.exe