rpm: set exec bits on plugins

rpm: set exec bits on plugins

Install plugin shared libraries with permissions set to 0755
instead of 0644 during RPM builds.

When building with rpmbuild, if a compiled file is not executable,
it's source files and symbols are not included in the vpp-debuginfo RPM.
This makes debugging problems with DPDK (and other plugins probably)
difficult unless you are running on the machine where the build
was generated.

Change-Id: I9cc92af855a66366ec3b81a87b82bfd9994bc180
Signed-off-by: Matthew Smith <mgsmith@netgate.com>