kbuild: fix Kconfig prompt of CONFIG_HEADERS_CHECK

Prior to commit 257edce6 ("kbuild: exploit parallel building for
CONFIG_HEADERS_CHECK"), the sanity check of exported headers was done
as a side-effect of build rule of vmlinux.

That commit is good, but I missed to update the prompt of the Kconfig
entry. For the sake of preciseness, lets' say "when building 'all'".
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

lib/Kconfig.debug

@@ -306,11 +306,11 @@ config DEBUG_FS
 	  If unsure, say N.
 
 config HEADERS_CHECK
-	bool "Run 'make headers_check' when building vmlinux"
+	bool "Run sanity checks on uapi headers when building 'all'"
 	depends on !UML
 	help
-	  This option will extract the user-visible kernel headers whenever
-	  building the kernel, and will run basic sanity checks on them to
+	  This option will extract the user-visible kernel headers when
+	  building the 'all' target, and will run basic sanity checks on them to
 	  ensure that exported files do not attempt to include files which
 	  were not exported, etc.