kexec-tools: use make_install

use consistent build flags  %make_install to have the benefits like
enabling parallel building automatically.

Signed-off-by: Coiby Xu <coxu@redhat.com>
Reviewed-by: Philipp Rudo <prudo@redhat.com>
Reviewed-by: Dave Young <dyoung@redhat.com>

kexec-tools.spec

@@ -159,7 +159,7 @@ autoreconf
 %endif
     --sbindir=/usr/sbin
 rm -f kexec-tools.spec.in
-make
+%make_build
 
 # makedumpfile
 make -C eppic-%{eppic_ver}/libeppic