remove RPM_BUILD_ROOT from asm-offsets.h

No file in rpm binary package should have the RPM_BUILD_ROOT string in it. To simplify building of external modules, our kernel-source package contains some temp files from the Kbuild system. asm/asm-offsets.h is one of the files that contains the absolute path if make O=$O is used.

* This file was generated by /var/tmp/kernel-source-2.6.14_rc4-build/usr/src/linux-2.6.14-rc4-2/Kbuild

Remove the $RPM_BUILD_ROOT string in the shipped tempfile.

---
 Kbuild |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6.18-rc2/Kbuild
===================================================================
@@ -28,7 +28,7 @@ define cmd_offsets
 	 echo "/*"; \
 	 echo " * DO NOT MODIFY."; \
 	 echo " *"; \
-	 echo " * This file was generated by $(srctree)/Kbuild"; \
+	 echo " * This file was generated by Kbuild"; \
 	 echo " *"; \
 	 echo " */"; \
 	 echo ""; \