eal: restrict empty declarations to doxygen

No need for those forward declarations (which breaks build when asking for
C++11 or adding pedantic flag).

Signed-off-by: David Marchand <david.marchand@6wind.com>

lib/librte_eal/common/include/generic/rte_cpuflags.h

@@ -47,12 +47,16 @@
 /**
  * Enumeration of all CPU features supported
  */
+#ifdef __DOXYGEN__
 enum rte_cpu_flag_t;
+#endif
 
 /**
  * Enumeration of CPU registers
  */
+#ifdef __DOXYGEN__
 enum cpu_register_t;
+#endif
 
 typedef uint32_t cpuid_registers_t[4];
 
@@ -100,8 +104,10 @@ rte_cpu_get_features(uint32_t leaf, uint32_t subleaf, cpuid_registers_t out);
  *     0 if flag is not available
  *     -ENOENT if flag is invalid
  */
+#ifdef __DOXYGEN__
 static inline int
 rte_cpu_get_flag_enabled(enum rte_cpu_flag_t feature);
+#endif
 
 /**
  * This function checks that the currently used CPU supports the CPU features