.gitignore: drop include/asm from ignored file list

Commit bb02c536 stopped creaing a symbolic link include/asm.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/include/.gitignore b/include/.gitignore
index 7cd3e90..bf142fc 100644
--- a/include/.gitignore
+++ b/include/.gitignore
@@ -1,5 +1,4 @@
 /autoconf.mk*
-/asm
 /bmp_logo.h
 /bmp_logo_data.h
 /config.h