meson: Address missing native directive in add_languages()

Specify `native: false` as we're testing a library against which the
tests must link.

Resolves the following:

```
meson.build:17: WARNING: add_languages is missing native:, assuming languages are wanted for both host and build.
```

Change-Id: Ie3173489c8738a76077e5f278bb0eef0d3697de1
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 032ef59..4cab166 100644
--- a/meson.build
+++ b/meson.build
@@ -16,7 +16,7 @@
 )
 
 if get_option('tests')
-    add_languages('cpp')
+    add_languages('cpp', native: false)
 endif
 
 # For memmem() in src/msgbuf.h