Making opus-uninstalled.pc use the .la instead of the .a

This makes it possible to link dynamically with an uninstalled libopus.
It does require the executable to use libtool however.
diff --git a/opus-uninstalled.pc.in b/opus-uninstalled.pc.in
index 3f2d674..19f5c93 100644
--- a/opus-uninstalled.pc.in
+++ b/opus-uninstalled.pc.in
@@ -8,5 +8,5 @@
 Version: @VERSION@
 Requires:
 Conflicts:
-Libs: ${libdir}/libopus.a @LIBM@
+Libs: ${libdir}/libopus.la @LIBM@
 Cflags: -I${pcfiledir}/@top_srcdir@/include