.gitignore: add LTO generated file

Currently, keep-syms-lto.c is being generated if LTO is enabled but unlike
keep-syms-lto.o it is not being ignored, so lets add keep-syms-lto.* to
.gitignore.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>

.gitignore

@@ -72,6 +72,7 @@ fit-dtb.blob*
 /Test*
 /capsule.*.efi-capsule
 /capsule*.map
+/keep-syms-lto.*
 
 #
 # Generated include files