Mark in-header function definitions as "inline"

Otherwise, generating the `clientdata_prop` modules will duplicate symbols,
leading to linker errors if the resulting modules are linked (or possibly
loaded) simultaneously.

Author: sethrj

Examples/test-suite/clientdata_prop_a.h

@@ -6,7 +6,7 @@ class A {
 
 typedef A tA;
 
-void test_A(A *a) {}
-void test_tA(tA *a) {}
+inline void test_A(A *a) {}
+inline void test_tA(tA *a) {}
 
-tA *new_tA() { return new tA(); }
+inline tA *new_tA() { return new tA(); }