[gnome-builder] build: specify libprefix for python

[Christian Hergert <chergert src gnome org>]

commit 0039b9832c5c8dd9a499668c2d03cc88435e07d1
Author: Christian Hergert <chergert redhat com>
Date:   Tue Aug 7 17:40:39 2018 -0700

    build: specify libprefix for python

 build-aux/flatpak/org.gnome.Builder.json | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 050b0b720..2f7daace5 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -837,6 +837,7 @@
                 "-Dctags_path=/app/bin/ctags",
                 "-Dfusermount_wrapper=true",
                 "-Dwith_tcmalloc=true",
+                "-Dpython_libprefix=python3.7",
                 "-Denable_tracing=true",
                 "-Dwith_help=true",
                 "-Dwith_channel=flatpak-nightly",