biology/paml: fix the build with LLVM 11
LLVM 11 switched from -fcommon to -fno-common by default. This revealed a
slight misposition of names in paml.h, which caused one symbol per enum
that's not used anywhere.
The fix just moves the names back so that the enums are named that, rather
than creating new unused symbols.
Reviewed by: zeising
Approved by: jrm (maintainer)