gdb: clean up some things in features/Makefile

- 32bit-segments.xml is not listed in the Makfile, but it's used.  This
  means that it wouldn't get re-generated if we were to change how C
  files are generated from the XML.  It looks like it was simply
  forgotten, add it.

This fixes PR build/28921.

(cherry picked from commit 1056aa3919)

gdb/features/Makefile

@@ -222,6 +222,7 @@ FEATURE_XMLFILES = aarch64-core.xml \
 	i386/32bit-mpx.xml \
 	i386/32bit-avx512.xml \
 	i386/32bit-pkeys.xml \
+	i386/32bit-segments.xml \
 	i386/64bit-avx512.xml \
 	i386/64bit-core.xml \
 	i386/64bit-mpx.xml \