Add /*build*/ to .gitignore

A common pattern is to build the source artifacts within a *build* dir.

For example, the wiki instructions for [building `wireshark` on `macOS` specify to
create a `./build`
directory](https://gitlab.com/wireshark/wireshark/-/wikis/BuildingAndInstalling#macos):
```
mkdir build && cd build && cmake ../ && make
```

So this commit adds this directory to `.gitignore` to prevent
accidentally committing build artifacts.

The specific format `/*build*/` protects against the following:
1. The leading slash makes sure only top-level directories that match
this pattern are ignored.
2. The trailing slash makes sure that only directories (and not files)
are ignored.
3. The wildcards catch all the various variations on `build` folder name
that are used...
9 jobs for master in 44 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #759751844
build:debian-stable

00:36:23

passed #759751839
build:rpm-centos-7

00:32:35

passed #759751842
build:rpm-fedora

00:38:00

passed #759751840
build:rpm-opensuse-15.1

00:39:57

passed #759751837
clang-10

00:30:06

 
  Test
passed #759751848
test:debian-stable

00:02:14

passed #759751845
test:rpm-centos-7

00:01:49

passed #759751847
test:rpm-fedora

00:01:48

passed #759751846
test:rpm-opensuse-15.1

00:04:40