Make sure libtheoraenc do not need libtheoradec

[petterreinholdtsen]

Addresses issue reported in https://bugs.debian.org/923940 about some symbols being unresolved.

[rillian]

The debian bug is already fixed on the master branch with this line in lib/Makefile.am

libtheoraenc_la_LIBADD = libtheoradec.la

which declares the transitive dependency in the ELF headers:

$ ldd _inst/usr/local/lib/libtheoraenc.so
	linux-vdso.so.1 (0x0000ffff9abfd000)
	libogg.so.0 => /lib/aarch64-linux-gnu/libogg.so.0 (0x0000ffff9ab5f000)
	libtheoradec.so.1 => /lib/aarch64-linux-gnu/libtheoradec.so.1 (0x0000ffff9ab36000)
        [...]

Nevertheless we might want to still do this, since info.c is the only remaining theoradec code not already duplicated. If so we'd need to also remove the line above update pkgconfig to drop the requirement.