libsysprof: add inside guards to prevent stray includes

This requires that things use #include <sysprof.h> now instead
of individual includes. A few cases are still allowed in the
libsysprof-capture library to make copy/paste embedding easier.

src/libsysprof/sysprof-proc-source.h

@@ -20,6 +20,10 @@
 
 #pragma once
 
+#if !defined (SYSPROF_INSIDE) && !defined (SYSPROF_COMPILATION)
+# error "Only <sysprof.h> can be included directly."
+#endif
+
 #include "sysprof-version-macros.h"
 
 #include "sysprof-source.h"