samples: posix: uname: add Makefile to compile on POSIX OS

Previously the sample was using some headers that aren't
available to the host, now we can add a `Makefile.host` to
compile the example on a POSIX OS like Linux:

```
# Go to the sample dir
cd ${ZEPHYR_BASE}/samples/posix/uname

# Compile the sample
make -f Makefile.host

# Run the binary
./build/uname

sysname[65]: Linux
nodename[65]: LAPTOP-YC
release[65]: 5.10.16.3-microsoft-standard-WSL2
version[65]: #1 SMP Fri Apr 2 22:23:49 UTC 2021
machine[65]: x86_64
```

Signed-off-by: Yong Cong Sin <[email protected]>

samples/posix/uname/Makefile.host

@@ -0,0 +1,5 @@
+# This makefile builds the sample for a POSIX system, like Linux
+
+uname: src/main.c
+	mkdir -p build
+	$(CC) $^ -o build/$@