Make required *-devel packages arch-specific

As required by
http://fedoraproject.org/wiki/Packaging:Guidelines#Requires_2
and suggested by Michael Schwendt at
https://bugzilla.redhat.com/show_bug.cgi?id=1199693#c16

package/rpm/pcp-pmda-cpp.spec

@@ -50,7 +50,7 @@ Metrics Domain Agents (PMDAs) for Performance Co-Pilot (PCP) in C++.
 Summary: Development headers for the PMDA++ library
 Group: Development/Libraries
 Provides: %{name}-static = %{version}-%{release}
-Requires: pcp-libs-devel
+Requires: pcp-libs-devel%{?_isa}
 
 %description devel
 PMDA++ is a header-only library that allows developers to write Performance