gems: Avoid flexible array member in Cached_font.

Switch the code to 0 lengh array instead. The code in
Lru_cache::element_size calls sizeof on this structure. This works in
gcc, but fails when using clang. Even for GCC however the documentation
states:

  "Flexible array members have incomplete type, and so the sizeof operator
   may not be applied. As a quirk of the original implementation of
   zero-length arrays, sizeof evaluates to zero."

Basically its an implementation quirk that clang does not support. Both
GCC and clang do support zero sized arrays however so using them here
allows both compilers to process this code.

Ref: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Issue #4421

repos/gems/include/gems/cached_font.h

@@ -30,7 +30,7 @@ class Genode::Cached_font : public Text_painter::Font
 
 		struct Cached_glyph : Glyph, Noncopyable
 		{
-			Glyph::Opacity _values[];
+			Glyph::Opacity _values[0];
 
 			/*
 			 * The number of values is not statically known but runtime-