[libpeas] Do not build tests when introspection is disabled/missing

[Christian Hergert <chergert src gnome org>]

commit 53a4f19887d9ae4e77f32dd6d71c8a7b516b4d5d
Author: Andreas Müller <schnitzeltony gmail com>
Date:   Mon Aug 10 21:53:33 2020 +0200

    Do not build tests when introspection is disabled/missing
    
    Fixes:
    | ../libpeas-1.26.0/tests/libpeas/introspection/meson.build:47:0: ERROR: Unknown variable "libpeas_gir".
    
    Signed-off-by: Andreas Müller <schnitzeltony gmail com>

 meson.build | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 8322fea3..43fb062f 100644
--- a/meson.build
+++ b/meson.build
@@ -254,7 +254,9 @@ subdir('loaders')
 if build_demos == true
   subdir('peas-demo')
 endif
-subdir('tests')
+if generate_gir == true
+  subdir('tests')
+endif
 
 summary = [
   '',