Add build requires alsa-lib-devel (rfbz#4820)

ffmpeg.spec

@@ -83,6 +83,7 @@ Source0:        http://ffmpeg.org/releases/ffmpeg-%{version}.tar.xz
 Requires:       %{name}-libs%{?_isa} = %{version}-%{release}
 %{?_with_cuda:BuildRequires: cuda-driver-dev-%{_cuda_rpm_version} cuda-misc-headers-%{_cuda_rpm_version} cuda-drivers-devel%{_isa}}
 %{?_with_libnpp:BuildRequires: cuda-cudart-dev-%{_cuda_rpm_version} cuda-nvcc-%{_cuda_rpm_version} cuda-misc-headers-%{_cuda_rpm_version} cuda-npp-dev-%{_cuda_rpm_version}}
+BuildRequires:  alsa-lib-devel
 BuildRequires:  bzip2-devel
 %{?_with_faac:BuildRequires: faac-devel}
 %{?_with_fdk_aac:BuildRequires: fdk-aac-devel}