meson: Fix typo in speex build file

Bad backport, host_system is not defined in this branch.
diff --git a/ext/speex/meson.build b/ext/speex/meson.build
index c87ef6a..dec656c 100644
--- a/ext/speex/meson.build
+++ b/ext/speex/meson.build
@@ -6,7 +6,7 @@
 
 speex_dep = dependency('speex', version : '>=1.1.6', required : false)
 windows_args = []
-if host_system == 'windows'
+if host_machine.system() == 'windows'
   windows_args = ['-DWIN32']
 endif