mail/eps: fix build on powerpc with clang

Add -fPIC:
ld: error: can't create dynamic relocation R_PPC_ADDR16_LO against local symbol in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output

defined in mime.o
referenced by mime.c

mime.o:(mime_content_type)