Material: Fix the build for c compiler (#5879)

- Mark inlined functions as static inline to fix the linkage for c compilers
- closes https://github.com/assimp/assimp/issues/5875

include/assimp/material.h

@@ -1557,7 +1557,7 @@ ASSIMP_API C_ENUM aiReturn aiGetMaterialFloatArray(
 * @return Specifies whether the key has been found. If not, the output
 *   float remains unmodified.*/
 // ---------------------------------------------------------------------------
-inline aiReturn aiGetMaterialFloat(const C_STRUCT aiMaterial *pMat,
+static inline aiReturn aiGetMaterialFloat(const C_STRUCT aiMaterial *pMat,
         const char *pKey,
         unsigned int type,
         unsigned int index,
@@ -1582,7 +1582,7 @@ ASSIMP_API C_ENUM aiReturn aiGetMaterialIntegerArray(const C_STRUCT aiMaterial *
  *
  * See the sample for aiGetMaterialFloat for more information.*/
 // ---------------------------------------------------------------------------
-inline aiReturn aiGetMaterialInteger(const C_STRUCT aiMaterial *pMat,
+static inline aiReturn aiGetMaterialInteger(const C_STRUCT aiMaterial *pMat,
         const char *pKey,
         unsigned int type,
         unsigned int index,