tests: Add -rdynamic to dwfl_proc_attach_LDFLAGS

tests: Add -rdynamic to dwfl_proc_attach_LDFLAGS

dwfl-proc-attach uses (overrides) dlopen (so it does nothing).  This
seems to cause a versioned dlopen symbol to be pulled in when building
with LTO. Resulting in a link failure (when dlopen isn't integrated
into libc):

/usr/bin/ld: dwfl-proc-attach.o (symbol from plugin): undefined
reference to symbol 'dlopen@@GLIBC_2.2.5'
/usr/bin/ld: /usr/lib64/libdl.so.2: error adding symbols: DSO missing
from command line collect2: error: ld returned 1 exit status

Add -rdynamic to the LDFLAGS to add all symbols to the dynamic symbol
table for dwfl-proc-attach.

Signed-off-by: Mark Wielaard <mark@klomp.org>