gitignore: fix overzealous exclusion

This was excluding any subdirectory anywhere in the tree named build*,
rather than just at the root - thus anything in
meta/recipes-devtools/build-compare had to be forcibly added. Change the
line so that it only operates at the root of the repo.

(From OE-Core rev: 3a60e5dba0ea914b1bd7a2073c1977e1d8529541)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

.gitignore

 *.pyc
 *.pyo
 /*.patch
-build*/
+/build*/
 pyshtables.py
 pstage/
 scripts/oe-git-proxy-socks