meson: Remove single-use build_tests variable

Change-Id: I186367f84c569a1ef1a2c32466d18fbbc33bfd65
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 630cabe..f66753a 100644
--- a/meson.build
+++ b/meson.build
@@ -33,7 +33,6 @@
     language: 'cpp',
 )
 
-build_tests = get_option('tests')
 gpiodcxx = dependency(
     'libgpiodcxx',
     default_options: ['bindings=cxx'],
@@ -93,6 +92,6 @@
 subdir('service_files')
 subdir('src')
 
-if not build_tests.disabled()
+if not get_option('tests').disabled()
     subdir('tests')
 endif