Fix static GHCi build (#8270)

When Ryan merged in the atomics branch, he made atomic_inc and
atomic_dec EXTERN_INLINE functions, meaning their definitions
became part of object files in the non-threaded build, instead of being
inlined. As a result, this broke the linker for static GHCi, since it
couldn't properly resolve the now-externally defined functions.

Signed-off-by: Austin Seipp <austin@well-typed.com>

rts/Linker.c

@@ -1349,6 +1349,8 @@ typedef struct _RtsSymbolVal {
       SymI_HasProto(unlockFile)                                         \
       SymI_HasProto(startProfTimer)                                     \
       SymI_HasProto(stopProfTimer)                                      \
+      SymI_HasProto(atomic_inc)                                         \
+      SymI_HasProto(atomic_dec)                                         \
       RTS_USER_SIGNALS_SYMBOLS                                          \
       RTS_INTCHAR_SYMBOLS