patch 9.1.1713: filetype: fvwm2m4 files are no longer detected

Problem:  filetype: fvwm2m4 files are no longer recognized
          (after 9.1.1687).
Solution: Add a special case in m4 filetype detection (zeertzjq).

closes: #18146

Co-authored-by: Damien Lejay <damien@lejay.be>
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_filetype.vim

@@ -308,7 +308,8 @@ def s:GetFilenameChecks(): dict<list<string>>
     fstab: ['fstab', 'mtab'],
     func: ['file.fc'],
     fusion: ['file.fusion'],
-    fvwm: ['/.fvwm/file', 'any/.fvwm/file'],
+    fvwm: ['/.fvwm/file', 'any/.fvwm/file', '.fvwmrc', 'foo.fvwmrc', 'fvwmrc.foo', '.fvwm2rc', 'foo.fvwm2rc', 'fvwm2rc.foo', 'foo.fvwm95.hook', 'fvwm95.foo.hook'],
+    fvwm2m4: ['.fvwm2rc.m4', 'foo.fvwm2rc.m4', 'fvwm2rc.foo.m4'],
     gdb: ['.gdbinit', 'gdbinit', '.cuda-gdbinit', 'cuda-gdbinit', 'file.gdb', '.config/gdbearlyinit', '.gdbearlyinit'],
     gdmo: ['file.mo', 'file.gdmo'],
     gdresource: ['file.tscn', 'file.tres'],

src/version.c

@@ -724,6 +724,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1713,
 /**/
     1712,
 /**/