build: Use separate makefile for "trace/"

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
--
Changes in v2:

* Do not depend on "qemu-timer-common.o".
* Use "$(obj)" in rules to refer to the build sub-directory.
* Remove dependencies against "$(GENERATED_HEADERS)".

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/.gitignore b/.gitignore
index 3a41765..0e38169 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,10 +3,10 @@
 config-all-disas.*
 config-host.*
 config-target.*
-trace.h
-trace.c
-trace-dtrace.h
-trace-dtrace.dtrace
+trace/generated-tracers.h
+trace/generated-tracers.c
+trace/generated-tracers-dtrace.h
+trace/generated-tracers-dtrace.dtrace
 *-timestamp
 *-softmmu
 *-darwin-user