.gitignore: update .s pattern for case insensitivity

A recent commit changed the suffix of generated asm files to .pre.s.
https://review.coreboot.org/#/c/13943/

Update .gitignore to support case insensitive file systems.

Change-Id: If6948d6fd00e19b08583d050a8dcd31f0ad8796b
Signed-off-by: Ben Gardner <gardner.ben@gmail.com>
Reviewed-on: https://review.coreboot.org/14012
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/.gitignore b/.gitignore
index 9aa8c29..40ebcc7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,5 @@
 *.o
-*.s
+*.pre.s
 *.iso
 memtest
 memtest.bin