lowdown: fix musl build

Whenever building a shared library, we need to compile with -fPIC.
(This is sometimes implicitly added by the hardening flags, but not
always, e.g. for pkgsMusl.)

pkgs/tools/typesetting/lowdown/shared.patch

@@ -11,10 +11,11 @@ index 955f737..2c9532c 100644
  
  www: $(HTMLS) $(PDFS) $(THUMBS) lowdown.tar.gz lowdown.tar.gz.sha512
  
-@@ -101,6 +101,9 @@ lowdown-diff: lowdown
+@@ -101,6 +101,10 @@ lowdown-diff: lowdown
  liblowdown.a: $(OBJS) $(COMPAT_OBJS)
  	$(AR) rs $@ $(OBJS) $(COMPAT_OBJS)
  
++%.o: CFLAGS += -fPIC
 +liblowdown.so: $(OBJS) $(COMPAT_OBJS)
 +	$(CC) -shared -o $@ $(OBJS) $(COMPAT_OBJS) $(LDFLAGS)
 +