rpm.spec.in: Update deprecated [Build]Prereq tags

AFAICT, Requires and BuildRequires subsumed the functionality of Prereq
and BuildPrereq in RPM 4.0, and none of the platforms we support with
libjpeg-turbo 2.0.x has RPM < 4.4.

release/rpm.spec.in

@@ -43,7 +43,7 @@ Group: System Environment/Libraries
 Release: @BUILD@
 License: BSD-style
 BuildRoot: %{_blddir}/%{name}-buildroot-%{version}-%{release}
-Prereq: /sbin/ldconfig
+Requires: /sbin/ldconfig
 %if "%{_bits}" == "64"
 Provides: %{name} = %{version}-%{release}, @CMAKE_PROJECT_NAME@ = %{version}-%{release}, libturbojpeg.so()(64bit)
 %else