Avoid using the bindir macro in BuildRequires

This doesn't work for flatpak builds as bindir is redefined to be
/app/bin.

lensfun.spec

@@ -6,7 +6,7 @@
 Name:    lensfun
 Version: 0.3.2
 Summary: Library to rectify defects introduced by photographic lenses
-Release: 20%{?dist}
+Release: 21%{?dist}
 
 License: LGPLv3 and CC-BY-SA
 URL: http://lensfun.sourceforge.net/
@@ -41,7 +41,7 @@ Obsoletes: lensfun-python3 < %{version}-%{release}
 Obsoletes: lensfun-tools < %{version}-%{release}
 %endif
 # for rst2man, if INSTALL_HELPER_SCRIPTS != OFF
-BuildRequires: %{_bindir}/rst2man
+BuildRequires: /usr/bin/rst2man
 
 %description
 The lensfun library provides an open source database of photographic lenses and
@@ -170,6 +170,9 @@ popd
 
 
 %changelog
+* Wed Jul 17 2019 Kalev Lember <klember@redhat.com> - 0.3.2-21
+- Avoid using the bindir macro in BuildRequires
+
 * Wed Jun 19 2019 Rex Dieter <rdieter@fedoraproject.org> - 0.3.2-20
 - use %%python3_pkgversion
 - epel7: Obsoletes: python34-lensfun (#1721810)