Add missing BuildRequires on gcc-c++

configure: error: C++ compiler cannot create executables

References: https://bugzilla.redhat.com/show_bug.cgi?id=1606142
Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

rubberband.spec

@@ -9,6 +9,7 @@ URL:            http://www.breakfastquay.com/rubberband/
 Source0:        http://code.breakfastquay.com/attachments/download/34/rubberband-1.8.1.tar.bz2
 Patch0:         %{name}-1.5.0-mk.patch
 
+BuildRequires:  gcc-c++
 BuildRequires:  fftw-devel libsamplerate-devel libsndfile-devel
 BuildRequires:  ladspa-devel vamp-plugin-sdk-devel
 Requires:       ladspa