.gitattributes: make tabs in indentation a visible error

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
diff --git a/.gitattributes b/.gitattributes
index 429f8cd..cd3fde9 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -5,3 +5,4 @@
 *.in eol=lf
 *.am eol=lf
 *.sh eol=lf
+*.[ch] whitespace=tab-in-indent