[gegl] build: add to, rather than replace cflags_common on aarch64

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 52020c31f6b26f782848d875d0e6d44e46ccbe1e
Author: Øyvind Kolås <pippin gimp org>
Date:   Sat Jan 22 00:40:26 2022 +0100

    build: add to, rather than replace cflags_common on aarch64

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 931091789..e3864efb7 100644
--- a/meson.build
+++ b/meson.build
@@ -214,7 +214,7 @@ if host_cpu_family == 'x86_64'
 elif host_cpu_family == 'arm'
   arm_neon_flags = cc.get_supported_arguments(['-mfpu=neon'])
 elif host_cpu_family == 'aarch64'
-  cflags_common = cc.get_supported_arguments(['-mfpu=neon'])
+  cflags_common += cc.get_supported_arguments(['-mfpu=neon'])
 endif
 
 cflags_c   = cflags_common + cflags_c