drivers: oprofile: Avoids building driver from direct make command

Creates new Makefile to avoid building driver if
'make drivers/oprofile/' is called directly.

This driver is usually built from arch/$ARCH and seems to have no meaning building alone.

---
 drivers/oprofile/Makefile | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 drivers/oprofile/Makefile

new file mode 100644
index 000000000000..361867ec2338
@@ -0,0 +1 @@
+#Does nothing, since the source is called from arch/$ARCH/ tree.

2.19.0