CMake: Fix STM32F091XC Mbed targets GCC_ARM build

The linker file for the GCC_ARM toolchain was not
correctly listed.

Author: hugueskamba

targets/TARGET_STM/TARGET_STM32F0/TARGET_STM32F091xC/CMakeLists.txt

@@ -5,7 +5,7 @@ add_subdirectory(TARGET_NUCLEO_F091RC EXCLUDE_FROM_ALL)
 
 if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
     set(STARTUP_FILE TOOLCHAIN_GCC_ARM/startup_stm32f091xc.S)
-    set(LINKER_FILE TOOLCHAIN_GCC_ARM/stm32f091xc.ld)
+    set(LINKER_FILE TOOLCHAIN_GCC_ARM/STM32F091XC.ld)
 elseif(${MBED_TOOLCHAIN} STREQUAL "ARM")
     set(STARTUP_FILE TOOLCHAIN_ARM/startup_stm32f091xc.S)
     set(LINKER_FILE TOOLCHAIN_ARM/stm32f091xc.sct)