Makefile.uk: Condition TCB reserve on Kconfig

Previously musl's Makefile would unconditionally reserve a TCB size,
regardless of whether musl was selected for build in Kconfig.
This change makes this reservation happen only when musl is selected.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #54

Makefile.uk

@@ -37,7 +37,9 @@
 # Please refer to ./arch/Makefile.rules for more details.
 # For this version of Musl the size of "struct pthread" is 200.
 ################################################################################
+ifeq ($(CONFIG_LIBMUSL),y)
 $(eval $(call ukarch_tls_tcb_reserve,200))
+endif
 
 ################################################################################
 # On aarch64, newer versions of musl no longer expect a 16-byte reserved block