include/lvgl: rearrange for standard use of extern "C"

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

lib/gui/lvgl/lvgl_mem.h

@@ -7,12 +7,12 @@
 #ifndef ZEPHYR_LIB_GUI_LVGL_MEM_H_
 #define ZEPHYR_LIB_GUI_LVGL_MEM_H_
 
+#include <stdlib.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <stdlib.h>
-
 void *lvgl_malloc(size_t size);
 
 void lvgl_free(void *ptr);