dist/redhat: Fix RPM package build

Commit 12daaf5 ("dist/redhat: fix rpm build error") did not fix the
error, at least not on our Jenkins build machines.

Looking at the RPM build logs, we create the build directory:

+ cd /builddir/build/BUILD
+ mkdir build

but then change directory to "scylla-jmx-1.2.rc1":

+ cd /builddir/build/BUILD
+ cd scylla-jmx-1.2.rc1
+ mvn install

and therefore fail the copy:

+ cp dist/common/systemd/scylla-jmx.service.in build/scylla-jmx.service
cp: cannot create regular file 'build/scylla-jmx.service': No such
file or directory

I don't know why Takuya put the "mkdir" in the "prep" section but
something like this should unblock the build.

(cherry picked from commit c6edab5990)

dist/redhat/scylla-jmx.spec.in

@@ -16,12 +16,12 @@ Requires:       scylla-server java-headless
 
 
 %prep
-mkdir build
 %setup -q
 
 
 %build
 mvn install
+mkdir build
 cp dist/common/systemd/scylla-jmx.service.in build/scylla-jmx.service
 sed -i -e "s#@@SYSCONFDIR@@#/etc/sysconfig#g" build/scylla-jmx.service