Config.uk: Select libcompiler-rt for libcomplex

The cpow* functions provided by libcomplex make use of the `__mul?c3`
functions, implemented by libcompiler-rt, when building with either
Clang or GCC.
Therefore, this change makes musl unconditionally select libcompiler-rt
when building libcomplex.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #64

Config.uk

@@ -39,6 +39,7 @@ config LIBMUSL_AIO
 config LIBMUSL_COMPLEX
   bool "libcomplex"
   default n
+  select LIBCOMPILER_RT
 
 config LIBMUSL_CONF
   bool "libconf"