Rename misnamed changelog entries

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog.d/fix_hard_link_across_drives b/ChangeLog.d/fix_hard_link_across_drives.txt
similarity index 100%
rename from ChangeLog.d/fix_hard_link_across_drives
rename to ChangeLog.d/fix_hard_link_across_drives.txt
diff --git a/ChangeLog.d/mpi-window-perf b/ChangeLog.d/mpi-window-perf.txt
similarity index 100%
rename from ChangeLog.d/mpi-window-perf
rename to ChangeLog.d/mpi-window-perf.txt
diff --git a/ChangeLog.d/x509-subaltname-ext b/ChangeLog.d/x509-subaltname-ext.txt
similarity index 100%
rename from ChangeLog.d/x509-subaltname-ext
rename to ChangeLog.d/x509-subaltname-ext.txt