uml: add back CONFIG_HZ

avoid-overflows-in-kernel-timec.patch makes CONFIG_HZ necessary for a
successful build.  UML lacks a definition, so this patch adds one.  It also
changes the hard-wired definition of HZ to CONFIG_HZ.

Note: this patch is a good idea even in the absence of hpa's time fixes.

Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/Kconfig

@@ -68,6 +68,10 @@ config IRQ_RELEASE_METHOD
 	bool
 	default y
 
+config HZ
+	int
+	default 100
+
 menu "UML-specific options"
 
 config STATIC_LINK